YEeeeeeeeah! Epyx is back!
And a dedicated vertical cabinet... be still my heart!

You are making a good decision here. Vertical games on vertically mounted monitors completely changes the experience.
Question, do you really need 6 buttons? There are only a handful of 6 button vertical games...don't know if it's enough to merit the full gamut of buttons. 4 buttons would be enough IMO.
If you go with JLW's, Andy's got those octo restrictors for the UltraStik's that fit the JLW's. Personally I prefer octo gates for shmuppin', feels more fluid to me, and they also seem to work better for classic games than the square gates.
For art, how about an artist for your needs? I know there's a few around here...

I like the Satan's Hollow-esque theme. You already have a very cool ominous looking Aliens cab, so a Satan cabinet could pair well. Especially if you give it the blinged-out treatment you gave Aliens. Red lights?
Is it going to have side-art? Or are you going with the glowing window disc again?