Are you getting all 3?  Or just one?  I wouldn't pay more than 150 bucks for a gutted cab with just the coin door.
As for the PC, I just got an off lease 3.0ghz core 2 duo for 150 bucks off fleabay.  Core 2 duo is the best processor for MAME from what I gather.  A P4, though will work for just about anything you want to play, and you can get those form anywhere between $0 and $75
If cost is an issue, make a 2p cab.  4p are more expensive since you need more buttons, joys and a pricier encoder.  I would ditch the trackball, unless you LOVE trackball games.  I personally don't think its worth it, not many fun trackball games to justify the 60-100 bucks you would need to invest to get a trackball.
For encoders, get a 2p xin mo from paradise arcade or GGG, they are like 25 bucks, plug and play and easy.
Buttons/sticks, depend on personal preference, but you can find them pretty cheap at most vendors.
The monitor is important.  I would just go 21.3 inch LCD.  They are 4:3, plenty big enough and usually good quality and can be found for under 200 bucks.  I wouldn't mess with an arcade monitor unless having one is really important to you, they are heavy, hard to find, expensive and a PITA to mess with.  Just go LCD, they are cheap enough, easy to find and plug right in to your PC