Ballscrews that long are going to have to be large diameter or you will get lead whip at even moderate speeds. Also leadscrews and plasma cutting do not play well together. IF you can find them at less than the price of a small car you will face a difficult task getting them mounted properly and keeping them clean.
You might want to rethink the leadscrew design and drop back to the favorite standby:
Rack & Pinion with belt reduction or it's close cousin the long belt drive.
Tom CAUDLE
www.CandCNC.com