Hi Shoot - A few things to look at:
1) Direct drive to a rack and pinion will give you very high speeds and poor control for slow speeds, can be a notchy movement. You need to figure out the required speed range for the plasma then gear the motor to suit. Steppers run well between 0-500rpm then have small torque to 1000rpm. Direct drives run well 0-300rpm. If you figure out your gear ratio and then your machine speed at 300rpm I think you will be surprise how fast it is!! Most R&P are geared down 1:3 or 1:4...
2) Using bearings, nuts, bolts and commercial steel seems a low cost way to go but steel is not straight and bearings actually aren't cheap. They need eccentrics to adjust and all those bits add up to a cost that a round rail and car will be. So consider a commercial linear bearing system. Much better solution.
3) Because the steel is not straight the motor is usually mounted on a lever and spring so to keep the engagement constant. R&P's do jump out of engagement if they get light on engagement, lots of torque available!
4) Don't build or buy anything until you have finalised the CAD, identified suppliers and all costs to eliminate cost shock!!
Enjoy Peter