How long is this?

Well, obviously I'm here, and W_R is here, and JoeNobody is here, and Bwakel Ketsu is here, and Industrializeit, and several others have lurked and every so often posted on my big thread. B_Sulpher wrote one of the FAQs and has occasionally posted recently too.

Missables: very few. There is a pioneer town, much like the one in DQ4 DS, and in each form of the town you should talk to everybody day and night and chant smell with a thief to make sure you have all treasures. Don't know if you've ever played the NES DW3 or if this will be the first shot at the game and so if this is going to be a small spoiler, but at one point you need to gather six orbs to progress the story, and the orbs are what progresses the town's development, so each time you find an orb is when you should check the town. Other than the pioneer town, nothing missable comes to mind.

There are 110 TinyMedals, and they are used on the threshhold system like DS DQ4 as opposed to the currency system in DS DQ5. The top prize is at 100; the last 10 are just for show. I think one of the possibly missable things in the pioneer town earlier stages is one of these.

If you want to have as full an item bag as possible, don't use any of the personality books you find or the Zen Scrolls. A few of the books are purchasable in certain stores or drops from monsters, but the majority of them aren't renewable. Zen Scrolls: there's a way around using this. It's used to turn any character into a sage, but jesters can become sages for free, so if you want multiple sages, just put the character through jester to L 20 first. Zen Scrolls are technically unlimited, but the only place to find them beyond two fixed plot finds is as a 1:384 ground find on one specific pachisi board (parchisi is DQ5 DS TNT). I accidentally pulled one off that board, have three in my bag, and consider myself lucky. I'm not going for 99 of them.

In terms of hours, days, weeks, or months, yes. Many of all if you're going to go for 49,005 monster medals. They drop at 1:16 (actually the first bronze for each monster drops at 1:8, but after that it's 1:16), so you'll have junctures where you get really lucky against the odds and you might get perhaps a dozen or more in an hour's play but other times the odds will be mean to you and you'll be thinner than that.

I made a discovery last summer: I took bronze raven monster medals to 99, just to see what happened. Bwakel Ketsu had indicated that the game recognized that and silvers and golds got thicker, but I discovered that not to be the case. Bronzes kept dropping even after 99 and were simply dead non-things that didn't count. Because the odds within the 1:16 drops are 5:8 for bronze, 2:8 for silver, and 1:8 for gold, I was actually discouraged. That meant once 99 bronzes are obtained for a given monster, it's better than 50-50 that drops for that monster will be useless.

Well, in the extreme post-game you can trade 2 bronzes for one silver, so that means the last however many silvers can still be obtained with a mixture of natural drops and bronze trades such that every drop still means something, but once you're to 99 bronzes AND 99 silvers, there's no shortcut for golds. The idea of over 10,000 medals being 1:128 drops with 7/8 of your work being useless made me decide that I wasn't actually going for all 49,005. I decided I was going for 32,670, which is 99 x every bronze and silver, and just having whatever golds had dropped naturally to that point for each monster, which will probably be anywhere between 10 and 40 or so. So that's where I decided to draw my own personal line, sort of treating the golds like item drops in DQ5. It's one thing to get 999 of each monster in the Big Book and recruit and max level all possible ones, but it's another thing to take every single item drop, some at very long odds, to max also.
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Yes, my DW3 goal is insane.

Every class change puts the character back to level 1 even if you've previously leveled in that class, and yes, in your hypothetical mage to warrior back to mage, you'd then have 30 levels worth of stat regaining but no new spells.

The main hero doesn't ever change classes. If while button mashing at the class change spot you accidentally sequence the buttons wrong and tell the guy you're trying to change hero's class, he says "Fool! You no longer wish to be a hero? That is strictly forbidden!".

This actually creates a situation where if you put all the rest of your team through every other class, everybody EXCEPT the hero has a very comprehensive spell list, and the hero has what he has, which is many very good ones but a few it would be nice to have and he just never gets the chance to get them. The main one in this camp for me is bounce, which is very useful against certain late-game powerful enemies who throw explodet at you; I very commonly throw up bounce with all my support and let those explodets go right back where they came from, but when the hero is in the party, it's only 3/4 effective and hero just needs to absorb the damage.

Two spells hero gets that no one else does: healusall and thordain. They're quite powerful, so it's not like hero's spell list is crap, he just doesn't ever get a chance at the entire mage / cleric trees. Sage is nothing more than mage and cleric put together, with no spells unique that one of the other two doesn't get, but mage and cleric have completely mutually exclusive spell trees with absolutely nothing in common between the two. So another way around using Zen Scrolls in a certain way is to just do all of mage and all of cleric, though in the long run sage is the best of the three in terms of natural stats and equips, so you'll want sages rather than tricked out clerics or mages if given the choice.

That leads me right into the next answer: 999 is possible for HP and MP. 255 is the max for all the attribute stats. All five stats and both point totals have seeds; the most difficult of these to grind by far are the Mystic Nuts which are the MP boosters. Mystic Nuts are dropped by only two monsters and they're both not until the postgame; all the rest of the seeds including Life Acorns for HP are dropped by a variety of monsters throughout most phases of the game, so if you get one or two thieves going you can start in doing extra building of everything except MP nearly as early as you want.
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Yes, my DW3 goal is insane.

Zoma and medals: nope. I honestly don't know if he's available to be battled repeatedly or not, but there's an alternate way in the postgame to get his medals, three at a time, 1 each of gold, silver, and bronze all at once. Baramos definitely is only fought once, and his medals come from the same alternate postgame source.

Personalities: these ONLY matter as it pertains to stat gains at level-up. Certain personalities are much better than certain other ones. By far the best personality for females is sexy, as all five stats get bonuses and it has no area of weakness. The two personalities for males that have all five stats at at least even with one or two in bonus are lucky and quick, though I don't think hero can be either of these two initially. Lewd, sharp, smart, and a few others are pretty good though all have one stat that's actually below break even so there will be give and take in most personalities, and depending on which class is in question, certain personalities fit certain classes better than others.

To determine the hero's starting personality, there are a series of questions then a mini-scenario at the game's very beginning. Your answers determine which scenario you see, and what you do in the scenario decides the personality. This is all very complex, but there are several FAQs that cover it in detail, so refer to those for hero's personality. I'd say lewd is as good a choice as any for a male, if you go female hero, definitely go sexy.

Once your game is underway, you'll be given three built-in companions in the tavern, but you're free to never use them if you choose and make your own. When you make your own, the personalities are decided completely by the stats you end up with after creating the character. In B_Sulpher's FAQ, there's a section on what stat should be most for what class for what personality, and this is a starting point, but there's actually some variation within it where occasionally you'll get something unexpected. As near as we've all been able to figure out, very specific stat combinations yield different personalities in certain classes, with just enough random ones thrown in to keep you guessing. An example of this is female thief, which is possible but extraordinarily difficult to make sexy. I'll post this now then continue in in a subsequent post once I have a chance to go find what I'm looking for in my own huge thread.
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Yes, my DW3 goal is insane.

Here, and it was around post #210 on my thread. I knew it was in there someplace:

"So this morning I'm seting up some characters in Ruida's, and I go looking over the archives for how to make a sexy thief. I find a topic in which somebody did it, and posted the specific stats. they were:

STR:7
AGL:13
VIT:10
INT:8
LUCK:12

So I try over and over and over and over to get a sexy thief, and also to duplicate those specific stats. Finally after what seemed like in excess of 100 tries, and no sexiness, I finally duplicated those stats exactly. "Ah, quite the sexy type!". YES!!!! I'm here to tell you, it's HARD to duplicate a stat set exactly, because that fifth and final seed needs to give you the exactly correct value after you got a starting point that worked and got the rest of the seeds to cooperate, too.

So, this tells me that the formula in the FAQs to make sure this is the top value and then this is second, etc, is only very general and the actual specific stats are programmed to go with specific personalities, and that even one of these being even one off would have ruined the whole thing, as I had a few tries where just one of the stats was just one value off but the order of what had the most through the least was exactly the same. Example, that stat set but intel 9 rather than 8."

Me again, now. There has been one other stat set that anybody cared to specifically record that yielded a sexy thief, and it was posted by industrialzeit a couple posts later in my thread. It was:

HP: 13
MP: 12
STR: 8
AGL: 12
VIT: 10
INT: 9
LUCK: 11

Not sure if the HP and MP matter. I doubt it, as I wasn't paying that any attention when I was trying for my sexy thief with the other stat set. The two do have in common the descending order AGL, Luck, VIT, INT, STR, but I'm certain I had that general order with other numbers and got other personalities.

This is probably way more than you needed or wanted. I'll go back and look on your post one more time to see if there are any remaining questions I haven't yet touched on.
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Yes, my DW3 goal is insane.

Diligent is very good for strength and vitality, so your HP will go up nicely and you'll clobber people appropriately in battle. It's below average for luck, which is probably the least important stat at the end of the day, and it's also slightly below average for INT and AGL, so those are the ones you'll want to seed and the low INT might end up meaning you'll be delayed a level or two in learning spells here and there. Again, with the personalities, they've been characterized before on various threads as important enough to know a little something about and tweak them if you can, but not so very important that they'll ever be a dealbreaker.

The other thing to know is that most (but not all!) accessiories overlay a personality. If you have an accessory equipped, the personality that goes with that accessory becomes your active personality and you get its stat growth instead, then when you unequip it, you go back to your base personality. I didn't mention that before because I was already worried I was way too long as it was.
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Yes, my DW3 goal is insane.

There are two threads that you might to look over to learn absolutely everything you could ever want to know about all the mechanics of thieves, monster medals, pachisi boards, and so on. They'll possibly spoil some things, so depending on how much you want to know in advance, read accordingly.

http://www.gamefaqs.com/boards/450388-dragon-warrior-iii/54254508

and

http://www.gamefaqs.com/boards/450388-dragon-warrior-iii/53836019

On this second thread, which is mainly about pachisi boards and a certain possible hyper-level bug, post #18 is the biggie. The gist of the thief mechanic in the first thread is the stealing skill gets better every eight levels. It was around levels 32 and then 40 that I REALLY started noticing a difference in how often my thieves stole a wide variety of stuff. On that second tier from L8 through L15 and the third tier from L16 to L23 they mainly steal tokens from the token-carrying monsters with low drop odds plus your standard assortment of herbs. In and of itself, tokens aren't a bad early theft because they allow you more freedom to play more pachisi earlier.
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Yes, my DW3 goal is insane.