An old friend of mine is a builder, and he told me that if one is going to use ply-board and beams, then use 1/4 inch (think) ply and use 2×2’s for the beams. If you use thicker boards, you will need thicker beams to keep them in place, and that all gets heavy.
If your Sukkah is up on a deck, you may want to get an outdoor rug to prevent the (cold) breeze from coming through the floor beams. You can get them from Lowes or HD. Expect to pay around $70-$100 for about 12’x12′.