Rob,
You can use almost any non-solvent glue to glue the foam together - polyurethane glue (gorilla glue, probond), carpenters glue (Elmer's, Lepage's), PVA glue (Weldbond) or foam adhesive (Liquid Nails, PL adhesives). All work well for gluing it together.
With any layout, the purpose of the roadbed is less to minimize noise (although it is a side-benefit) than to raise the track above the level of the base board. If you look at real tracks, you'll see (for the most part) that they are built on a road bed of gravel, usually higher than the surrounding terrain. This is to provide drainage and keep the tracks from being washed away when it rains.
With foam construction, you can lay the track directly on the foam, and then carve away from the sides of the track to simulate raised roadbed, or you can use a roadbed product. On foam, I like to use Woodland Scenics TrackBed, a foam roadbed. Just glue it down where you want the tracks to go.
As for painting, I usually check the bargain aisle at the paint store, or the paint section of Home Depot for returned quarts of earth-toned latex paint. You get them cheap, and while they may have looked awful in the living room, they're a great base for scenery. If you have your landforms roughed in with foam, you can slap a thick layer of paint down, and while it's still wet, sprinkle ground foam on it. Give it a shot of wet water, and when it's dried the ground foam will have adhered to the paint - your first layer of ground scenery, and where the foam didn't stick looks like bare earth, because of the paint.