What is the projected cutting size of your table? Plasma only? Plasma, Ox/Fuel? Plasma, Router?
How heavy do you expect your gantry will be when complete with everything mounted?
Are you going to be cutting very thin material as well as thicker, say 1/2" steel? Stainless? aluminum?
If you get a good straight table (no binding on rails) & the gantry doesn't weigh 300# when completely outfitted. It doesnt take much to move it.
The problem is in the motors having the ability to stop the motion of the gantry in 1 direction & head back the opposite way without loosing steps (if you're using stepper motors). All the above questions pretty much need answered, or at least closest best guess. To help determine gear/drive ratios. The heavier it is & the more friction the motors have to overcome all comes into play.
If you have started your frame construction & have the ability to post a few pictures. That always helps others to understand your problems.