@v3rd1ct As you go through the wiki and look at controllers, one of the challenges you might find is that there is no wiki for keyboards. It's not really an oversight--several of us here have the combined details and first-hand experience to probably write one. In fact, I will probably take on the task at some point, but in the mean time, here is a good thread about it. Having a keyboard controller is great, especially if you plan to have a lot of buttons. My four-player setup would be a lot easier if I had just focused on a 2-player design, but IPAC is a good solution.
The other thing that will add to the complexity is that with your focus on fighter games, you might find that FBA emulates these better than MAME. So, suddenly you find yourself trying to manage multiple arcade emulators. Maybe you even decide to use the ARCADE folder for your ROMs to simplify the interface. Managing controls over various emulators is part of the challenge, but also part of the fun. It can be done, so hang in there.
I don't often say it out loud to people, but I have to admit that one of my hobbies is "configuring a DIY retro-gaming system". For me, it is never finished, and with new versions of Emulation Station on the horizon, new theme capabilities, updates to emulators, new Pi hardware. . . there is no end in sight, and I love it! But, I still need to write that keyboard controller wiki page.