I would recommend using 2" foam as a base instead of plywood since you are building a modular layout. It's lighter, easier to work with, and is plenty strong enough to support trains. You can use it for both levels (full disclosure - it's what I'm doing with mine which will be 20x20x4 around the room). I would use 1" thick plywood for supports though - a littler stronger than the Woodland Scenic foam supports. You can get away with placing them 12"-18" apart depending on the length of the run and other scenic considerations. As for the risers, use them in conjunction with the incline/decline starters - they come in multiple heights so you can get whatever height you desire. You may want to buy a second set with Fasttrack and place side-by-side - you may find a single set too narrow for your tastes.
-Greg
