Have you bought the motors already, and now are trying to design the machine around them? Usually it's better to go the other way around, building the machine first. Stepper motors with a lot of holding torque tend to have problems maintaining that torque at speed; it drops off rapidly.
The most important thing to consider when building a machine to cut stone or other abrasive material is protecting the moving parts from the dust (if you're cutting it dry) or slurry. )if you're cutting it wet). Have you looked into bellows construction?
A source of rack and pinion hardware that people here use a lot is CNCrouterparts.com . They have a system you can buy that reduces the speed of the pinion and spring-loads the pinion into the rack: PRO Rack and Pinion Parts | CNCRouterParts :