The 8700k processor is great if you can afford it, because not only you are going to play games but use your PC on a daily basis for numerous tasks. But keep in mind that CPU uses the new chipset that also uses dd4 ram. So by picking that CPU you gotta change the ram to dd4 with at least 2666 clock to make it worth your while. And if you are in between GPUs the integrated will help you fill that gap for a while, granted its no super GPU but its there and it works since can handle dx12. Keep in mind this CPU needs win 10 to work same as the mobo you pair it with. So if you are on Win7 now, upgrade the old kit to 10 so when you change parts and boot the PC can recognize them fine and not get stuck on reboot loop if you have Win7 .
As for the core dilemma, technology is going only one way, forward. So having the extra cores now doesnt mean they will get redundant in the near future.
Ram 2x8 dd4 2666 should be fine, because at least from my personal experience some ram sectors tend to "burn" over time and a single 16gb means that whenever the task you are performing pass for the damaged block the PC will either BSOD or stutter or hang, etc.
Am not familiar with AMD but their CPUs are also fine both in price and performance, its only a matter of branding, if you are familiar with Intel and know how they function you are going to pick Intel, same with AMD.