Welcome to the active life on the forums! Lots of great people around here, and as you obviously are aware, TONS of useful information!
Something else you could consider for your build, would be finding the Ms Pac cocktail schematics by Kyle Lindstrom. I guess it would be more for the ambitious/crafty types, but at least you could say you built it from scratch.
I like what you're thinking about the button layout. Vertical games don't often use more than 4 buttons. If you check out
MAWS' Deluxe Search, and sort by vertical/X buttons, only 22 games use 5 buttons, 76 games use 4 buttons, 272 use 3 buttons. If any of those 76 4-button games appeal to you, you could try for a diamond layout with your buttons, and shouldn't lose too much space...
The biggest downside i've come across with a cocktail cab, is you're gonna miss out on the 2p shmups, like 1942, etc.
Edit: You should drop the buttons on the front panel. You'll have a better experience with sticking to coin doors for the coin switches, be it mechanisms, or just re-locate the coin buttons to the coin door.