Regarding the R&P v Ballscrew, both have their pros and cons.
As Lee mentions, a plasma doesn't really need the accuracy of ballscrews, but the biggest drawback is coming up with a design to keep dust out, as I'm sure you're well aware, plasma dust is pretty abrasive and will quickly kill ballscrews.
However R&P on the other hand, provided it's designed well enough, will be relatively unaffected by dust, but you lose accuracy, and have the issue of dealing with wear, especially uneven wear which will be challenging to adjust out to minimise backlash while avoiding binding. Off course the solution to this could be simply changing the racks when worn, building in some form of spring loading of the R&P, or using anti-backlash pinions.
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Both options have pros and cons, both will work, and both can be designed to have long service lifes. As with most things, it comes down to implementation.