You want:
1. Gen2 cabinet that is in good shape (the one with the led accent lighting around speakers, cash out/help button assembly.)
2. You want a MLD screen (these are the dual layered LCD's to play the "reel depth" games).
3. MPU, you want the 660 or a 625 with the discrete video card (the 660 already includes it).
4. Show bios is a must if you are going to load it with more than 15 games.
5. You want dynamic edge-lit buttons that are nice and bright.
6. The correct bill validator that is the actual up stack model, so you don't have to insert tickets upside down.
7. Get a tito system (you can easily add this later), but make sure you at least get a working printer with it.
8. Polygon topper with glitter bar (this is just for eye candy), but at least get a working Polygon Topper if you can't source one with the glitter bar.