I would not consider using timing belts on a table that large for plasma. If you did, you would want pretty large belting and then there goes any savings over R&P. I would suggest R&P for X and Y. Not sure that the electronics package will suit you for very long, but you would want the larger motors. That is about the size I have on my 4 x 4. I think most guys running a 4 x 8 table have more robust electronics than what you are looking at. Remember speed is critical with thinner materials on a plasma table. Or you will just be cutting plate.
Good luck with it.