First off, welcome aboard!
Your questions are a bit open ended, and there's really no definitive answer.
Check the WIKI for lots of info on front ends for MAME, as well as cab friendly juke software. There's tons to choose from and everyone has a favorite. Just really depends on what you're after.
And the same goes with cab design. To keep things simple for a first build, you might want to decide what type of games your prefer (vertical shooters, classics (pacman, galaga, etc), sports, etc etc).
Once you've settled on a genre, you can usually determine pretty quickly the basic controls you'll need, usually 1 stick and 4-6 buttons per player, 2 players. Maybe add a trackball, and possibly a spinner. There's lots of good reference stuff in threads about good layouts. Do some searches and pay particular attention to stickied threads, theyll give you a lot to go on.
Take your time, do some planning and enjoy the process.
Or.... slap something together cheap and quick so you can get your game on, then you'll quickly see what you f'd up and what you got right and refine accordingly.
It all depends on your style!