Hollow-core doors, $19.99 each from Home Depot, made benchwork construction a breeze.  I fastened the doors to a 1" pine ledgerboard along the walls and used knee braces for support.  I designed my benchwork to be portable in case we ever have to move.  The layout sections are simply lag-bolted together as shown in the photo.

The resulting benchwork is quite rigid and easily supports my body weight.

I used green foamboard insulation from Lowe's as a scenery base.  I initially used three inches of foamboard, which resulted in a track height of 53" off the floor.  Upon placing temporary track and structure mock-ups, I found that 53" was a tad too low for my taste.  I added another 2" layer of foam to bring the benchwork height to 55".  I'm 5' 10" and 55" is about armpit height.  I tested operations again before permanently attaching the additional foam layer, just to make sure that I could manually uncouple cars anywhere on the layout.  The resulting 5" foam base layer allows for plenty of scenery below track level.

I used a hand saw rather than a utility knife for cutting foam, cleaning up the edges with a drywall sanding block.

PL300 is a great adhesive for attaching foam.