Check this site for linear motion products. The link points directly to their version of Hiwin linear rails. overstock & surplus automation products: Linear Bearings/Rail, BLH Linear Guideway
I wouldnt recommend Mach 3 for your software. Consider UCCNC for control software and a compatible motion controller like the UC300 or UC400 series. I havent done price comparisons lately but I think it is less expensive than MACH3 and by all accounts better. I built my first router using a Gecko G540 driver and a USB100 controller from UCCNC. Pretty inexpensive components, and that machine was comparable to your design in overall size. That's another option.
If your considering ballscrews over rack and pinion due to backlash concerns dont. Each drive system has theirs pro's and con's but the backlash reputation that seems to haunt rack and pinion systems is pretty much non-existent these days. I used Moore Gear for my gear rack an 2 routers and neither had any backlash problems.
Good luck with your build.