I have never liked leadscrews in a plasma cutting environment. Not only do you have the problem of critical speed (whip), but keeping the screw threads clean is difficult. Although many other drive types, chain, toothed belt, and friction can be used, I prefer gear rack and pinion. R & P is tolerant of moderate amounts of dirt, and can be setup with spring loading so as to yield zero backlash. We built one 4' wide machine, the Model 610A, where we used a single X axis drive motor with a 4+ foot 3/8 inch shaft having a pinion on the left side and the second drive pinion on the right. We used 3/8 gear rack placed teeth-up so that the spring laded pinions meshed with the rack on left and right sides. While not the ideal in terms of keeping it clean, it was simple.
With a decent "delta" carriage design you can operate quite successfully with single side drive on a plasma machine.
Getting a 3/8 or 1/2 inch shaft should be easy from your near steel service center. For gear rack, an excellent source is Standard Steel Specialties [www.stdsteel.com/gear.htm]. We used 1/2" x 1/2", 24 dp, 20 degree pressure angle rack and pinions. In choosing pinion number of teeth, be sure to have at lease 18 teeth on each pinion for smooth operation.
I'd use rack for the cross axis as well, and it can be easily used for the Z axis.
There are lots more aspects to a good design.