If you're gonna spend the cash on the ipac use it for P1, P2 and all your extra admin & coin buttons.
Then jump on ebay & buy 2 cheapo knockoff playstation style usb controllers for about $4 each, hack them as you would the 360 controllers for P3 & P4.
The 360 controllers would be unnecessarily expensive imo.
Don't know of games that use 6 buttons for P3 & P4 but i've never looked.
I don't have that many friends.

As for the screen, it depends on your design I guess.
Personally I much prefer having the TV simply sit on a shelf with its case still on. I don't have to worry about decasing it & mucking around with the power in it.
Plus it is a lot easier when it comes time to move the arcade around.
My one has been in half the rooms in the house and even at a couple of mates places over the past year, I can simply pickup the TV & PC and take them out each time it has to move.