I promise I will stop asking daft question soon. Ish.

[Jonathan]

1) Depends how you use it, but I'd reccomend using something larger than 30x30 for the gantry - don't worry about the weight. It's easier to mount the SBR20 rails on something that is at least as wide as the rail base.
2) MDF wouldn't be my first choice, but it will still add stiffness.
3) Yes it will be fine on the 80mm surface.
4) 6 of one... could spend forever discussing this. I'd probably go for two motors, Jazz will say one motor and a long belt. Price wise there's not that much in it.
5) Use 16mm diameter, 10mm pitch (RM1610) ballscrews, not 20mm diameter. You wont gain anything by using the larger size ballscrew since those motors, especially when run from only 48V, will not be able to get good accelleration and speed with the larger ballscrew, especially if you're thinking of the RM2005 from Chai. Get RM1610. Yes the critical speed is lower, but that's only an issue on the 1.5m ballscrew and even so if comparing to 5mm pitch screws it will still go much faster as the rpm is halved for a given feedrate. For the Z-axis the best option is RM1605. Also you should use timing belts/pulleys to get the best performance...couplings are notorious for snapping or being annoying to align, whereas belts are trivial to align, allow you to change the ratio to gain speed or resolution and also help suppress resonance.

Oh, and one last question does Chai sell the water cooled spindles?

Yes, but he's one of the more expensive sellers for spindles on eBay. I got mine from solar.jean on eBay, and know a fair few other people who have and it's been excellent.

no mounts though :(