For ruined buildings, I'd highly recommend
Amera Plastic Mouldings (even if he can't spell "moldings" correctly

). Great inexpensive vacuformed terrain at a very good price!
I've also been looking at PSG's Warcore buildings, but they seem to be out of most of them. I hope that isn't permanent.
Another great option, although not as affordable as that from PSG and Amera, is
Flying Tricycle's laser-cut wood buildings, including a stackable unit that you can unstack in order to fight from floor to floor. Beautiful stuff; my FLGS has some of their older fantasy stuff painted up for use in exhibition games.
Litko Aero also does buildings, but they're more a modern look. They also have their catwalk/platform system, which might let you put sky bridges between buildings.