Better to beef up the under-suporting structure, then you can utilize a thinner "roadbed" board/plywood, maybe as little as 1/4" lauan. Maybe 1 X 3 or 1 X 4 for the vertically-oriented support beams, assuming you will be using fir/spruce "construction" lumber. When picking out be sure it is straight, no knots, best if plane-sawed since the grain will be vertical in your structure. Hardwood lumber would offer better/stronger support at the dimensions you've given. Remember also that most lumber from the yard will be "wet" (14-20%) even though it is kiln-dried, and therefore may cup, bow, warp, etc. Best to let it climatize and stabilize somewhat for two weeks or more indoors to see what it is likely to do.
Wally
