Probably best find the docs for the emulators you're planning on using and see if they have any minimum or recommended specs listed. I know a lot of emulators have really come a long way in the last few years some of them might support really low-powered machines.

Anecdotally, I recently built a PC out of old bits and leftovers from friends' upgrades. I used it more for native PC games but it was an incredibly cheap way of playing anything that wasn't a recent AAA resource hog, so maybe see if people you know have parts that are just gathering dust.