Hi George,

I've built a couple of smaller belt drive machines so have a little experience and they work very well.!! . . . . . That said thou let me try and talk you out of it.!!!

It actually doesn't work out much cheaper than R&P or even cheap chinese ballscrews by the time you have bought the pulleys and good quality belt along with bearings,spindles etc. It's really doesn't lend it's self very well to large format machines due to the difficultys in keeping the belts correctly tensioned.

My experence was that even 3:1 wasn't enough reduction to have a good resolution and keep the motors working in a nice rev range.!! . . . And this was with relatively small pinion so smaller Dia = less pitch or travel per rev. . . . IE: 25mm Dia pinion x Pi = 78.5mm travel(pitch) per rev / 3 for 3:1 reduction = 26.1mm per rev.
So thats 25mm drive pulley with 75mm driven pulley.
This is quite a high pitch, the machine will go like hell and well above 30mtr/min but the resolution will be lower.! . . Not so big an issue . . . BUT . . The bigger problem comes when you want to go really slow like 500mm/min this will mean the motors only turning 20 RPM and at this speed the motors complain and you can get cogging affect.

If your only ever going to cut wood or plastics with a good quality spindle then an high pitch will be ok and will handle the high feeds ok.
But if you want better resolution or have a desire to cut materials that require lower feeds then a pitch more like 10-15mm per rev would probably be a better all round target.!!
This would however require 6:1 reduction and great big pulleys which are not cheap. Plus all the above figures revolves around using a small 25mm Dia pinion and what would more likely be the case given the size of machine and belt requirements is that the pinion would be larger which then means the driven pulley gets very large and expensive. (Unless you do multi stage reduction which gets complicated and more expensive.! smaller pulleys but more of them and extra belts)

IMO the best option regards cost/performance/maintenance/ease of build for large format machines is R&P.! . . OR. . If you want to spend a bit more money and time then go for the ultimate and use ballscrews with a rotating ballnut like Jonathan's machine.
My first choice would be Jonathans way every time because of the bennifits ballscrews give but if budget plays a part then I think you'll find that there's very little difference between R&P and belt drive done correctly.

Regards Z Axis I'd build it my self using ballscrews and belts but no reduction 1:1, this is to help with resonance and also makes build and motor placement easier and that bit neater.!
Best tip I'd give is don't skimp on the Z axis because it takes all the strain of cutting forces and no matter how good rest of machine if the Z axis flexs it means nothing.!!

I can help regards linear rails I'll send you a PM.

Hope this helps.!