If you want some good advice on plasma cutting machines, you may want to give mike a ring at Techserv (http://www.techserv.co.uk) They make some big plasma cutters, and mainly use analogue drivers and brushed servo motors for X and Y and stepper for height control (Z)
Noise is also an issue, so shielding is very important.


Also for long strokes, to keep the cost down look into using a rack and pinion.


I hope that helps.