1. A few layers of primer and then a few layers of paint rolled on is fine. Just remember to give it a sand between coats.
2. You will need some sort of ventilation. Is your cabinet perfectly air tight? I doubt it is, so you probably won't have to hack it up unless something cuts out for overheating.
3. I picked up some cheap $25 2.1 speakers from fry's. The two mains are pointed down at the players above the screen and the sub is inside the cab. They sound great. Are you looking for jukebox usability or just gaming? For a gaming cabinet, you really just need to make sure the speakers are angled at the players.
4.
http://mamemarquees.com/ is one of them
5. The software switching automatically is a great feature to have. Makes it completely user friendly for anyone. On my cab I have two 49-ways, purchased before U360 existed, but the auto joystick switching is great. I never have to know which joystick to use as it switches it for you.
If you have the money, get the U360. It is an investment in years of entertainment, not just some everyday purchase.