Hi Jaques,

Clives suggestion to use 10mm screws is correct and it mostly do everything you want, even small stuff. But if you really want flexabilty then connect motors to screws using Timing belt and pulleys with 1:1 ratio then you have option to change up or down if required. Also and much better reason to use Timing belts considering the drives your thinking to use is to lessen resonance which affects motor performance/smoothness.

I'd also urge you to use Profiled linear rails rather than cheap Round rail Chai sells. While Round rail does work ok the slight extra cost of profiled rails makes huge difference to machine stiffness and reliabilty. It will also increase sellabilty and value much more than the extra cost difference.

Also with Motors you'll notice No Difference between 3.1Nm and 4Nm motors. The important thing to look for is inductance you want Lowest inductance possible. This will allow higher RPM and more torque for same voltage compared to High inductance motor.
Again Clives advice of AM/EM drives and 60-70Vdc is sound advice you'd do well to take up. Esp when using Slaved motors.!

For Components contact Fred Lee here and get a quote. He's good to deal with and his stuff tends to be slightly better quality than Chai's. Esp the End bearings for the screws.
http://www.aliexpress.com/store/314742