In fact, farther than 50ft on an Ethernet wire you can forget about it. There will be too much noise and the signal strength will getting poor... But there's plenties of solutions for you. You can go buy another router and put it half way to extend your current lan or you can try the Powerline Ethernet technology.
I didn't make a game on PSP but if you want, you must install everything for homebrew. Otherwise, I don't think Sony will sell it's PSP's SDK to anybody else other than "real games developpers"... and if, I do not want to know the price for it.
First I would like to say that the Network Store and the tool they give are total mess ! There's absolutely no customer support. It's a real shame and I don't recommend you to buy anything with your credit card on that site.
So finaly, after spending nearly 3 hours of my "precious time", here it is :
1. uninstall everything/anything about Playstation(R) Network Downloader,
2. go download the tool here (I've got the setup v1.00.0003 and it install application v188.8.131.52)
3. Execute the setup where you downloaded it in a prompt like this :
This will force the setup to install in US/English language.
In my case I didn't reboot but because you are on Windows, you should ;)
4. IMPORTANT: Plug your PSP to your computer and get it registered/activated to the web site here. This part isn't very well describe but you must do it.
5. Go download and open any file with .XPD extension.
6. Let your firewall get the network downloader go to the internet