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Tbh, mame roms are the problematic ones, for the console emulators just torrent search for NES/Snes etc roms and look for collections, you can use sites like http://www.doperoms.com/toproms.php?system=super%20nintendo%20snes to download individual roms to test but you want a collection with all the roms organised.
Use this site https://github.com/retropie/retropie-setup/wiki/Nintendo-Entertainment-System to go through emulators and find the recommended ones to choose, if you go for a Pi 3 you can run pretty much all of the console emulators without issue.
Oh and to add, you should get a Pi 3 even if you don't need the extra power, the fact that it has built in wifi makes transferring roms way easier.
I've built an arcade cab using a Raspberry Pi, getting some of the mame emulators to work with arcade controls can take a bit of work but getting console emulators working through emulation station is pretty straight forward. I really like the look of the the NES Classic but if you're willing to put in a little effort it's way better to to go the Pi route, that way you can have Snes and megadrive games too. The main thing is downloading the correct rom packs for your chosen emulator.
Check out Floob on youtube, really good tutorials on retropie and emulators https://www.youtube.com/channel/UCdSu9Y0xAyY8V77vj6mQ-0g/videos