When I make my larger scenebuilds, I use PHX for everything.
So, for example, if you want to make a road, I would suggest starting out with the appropriately sized PHX plate, then using some 1xY plates, colored white and yellow, for lines. As far as materials go, try being creative with the in game ones. Recoloring your material coated PHX plates can kinda help them resemble something else. For example, you might try a plasterwall texture, on a plate colored black.
As Santz said, L4D props are good for props and stuff in your builds. HL2 is good for things like columns, debris, and decorations. HL2 also has some full size buildings. Lost coast has some great columns.