For the cutting area I have a plan to build a removable water table. It just needs to be a shallow pan and the slats can mount inside of it. If the mpcnc is built with the extendable legs then the machine can be raised high enough to slide the pan in and relevel.
I really dont know what issues are going to come up from heat. My plan is to address these as needed. I think that the water table will keep heat to a minimum in the work area. I think the z axis is the part that is the most susceptible to heat from the torch or material being cut.
The electrical noise issue is another thing that I plan on dealing with as I go. I helped a friend build a plasma table about 8 years ago and I remember we had some issues with grounding. Im pretty sure theres is build thread on pirate4x4 of his table. I should see if I can find it. The biggest problem you may have is with the cheap plasma cutter. Everyone I talk to about plasma tables recommends staying away from any plasma cutters that use high frequency to start the arc. I think all of the cheap chinese ones are HF.