You are right to be concerned. I have put together a torchmate, could never get it as tight as I would have wanted. If you do buy I would highly recomend incorporating a spring or air cylinder to tension the pinions against the rack. You may want to look into Bishop Wisecarver V-rails for an improvement to the bar stock rails also.
Have a good look at the design of the Dynatorch gantry for ideas, they have a 3D cad model on there site to view.
You can straiten out bowed steel pretty well if you string a taught wire up next to it for a reference. The gap between you measure with a caliper or can get fancy and rig up a jig with a continuity tester.
Thats all I can think of of the top of my head.