Typically R & P resist the dust & grit better than the ball screws. For ths size table you are suggesting, If your screws are long enough for you to improvise some sort of telescoping or self storing covers for them they would be great. That is providing you do the math & see if you can get the speed required to use plasma.
For gear rack: the 20 tooth 20 pitch is fairly common.
Probotics, I have only heard of them & noticed them being used in 1 build of a plasma table. (Not being negative, Just my personal observation)
Xylotex, I haven't used their products personally. They seem to have a great reputation with service after the sale.
For the size of your proposed table the 425 oz in NEMA23 steppers should be adequate.
I'm not sure about using any part of a CNC operated table for a welding bench. Someone else here I'm sure can answer that question for you.