You might want to look at some of these packages www.xylotex.com & www.hobbycnc.com Both of these are in my opinion still on the small side to make a table of the size you mention. www.candcnc.com have packages speciffically designed to operate larger tables in both stepper & servo motor power. They also carry many component parts which will allow you to source your own power supply, drivers, etc to build your own. If that's the direction you want to take. There are many other sources out there also.
Depends on lots of factors really. How heavy will your gantry be. How fast is it going to need to be, to cut the material(s) you want to cut.
Rest assured that 1 of CandCNC's packages will run a pretty large table at any speed needed for plasma cutting
No affiliation with any of the above vendors. just a satisfied customer of CandCNC.
If it works.....Don't fix it!