Here is my current idea;
US a raspberry pi has a server hosting all of the games
Retroarch as the interface
USB hub with 4 snes style controllers
I guess i am just worried this won't work/will fall apart? Anyone have experience with a setup like this?
In my experience retro gaming works better on the pi itself with RetroPie! Uses retroArch plus other emulators and has a great front end: emulation station. Even has Kodi embedded. Takes some time to configure but works great! You can also check /r/RetroPie for info and support
If you want to use the MiBox, just put the games on the internal memory; there should be space enough for retro games. In my experience retroArch on the MiBox was a bit buggy. I have better experiences with separate emulators per system. I like the .emu emulators because they're simple to use but powerful. If you want a single front end to browse and open your games (like with retroArch) you can use apps like Arc Browser, Nostalgia or Gamesome. Arc seems to be the best option but comes with a high price
Good luck setting up your retro game station!