Are you proposing fixed gantry/moving table or a moving gantry? If the expected workpiece is relatively lightweight (small amount of MDF, balsa wood, PCBs, etc.) I'd suggest a moving table design gives more bang for the buck so to speak. A moving gantry is going to be quite a lot heavier, harder to make rigid and will limit the performance with a small motor.

I concur on the 12x3 trapezoidal. Its not so expensive and even without an antibacklash nut will give a better result than M12 rod.

What about the spindle? Dremmel or something meatier?

Oh and on the stepper drivers, came across these... no idea how good or what chip, was thinking about getting one to find out but at £8.99/channel.... They also do a cheap (£30) 27v/13A PSU