the pieces are pretty durablecause i use a hard dental plaster, but i am planning on molding the finished parts to cast in resin. then the building should be super durable and lightweight too.
As for reusing the foam molds, i only use them once. i have reused them before but the warp easy and get hard to line back up. but since all the molds you see in the top picture came from a single $4 packet i figure it's good bang for the buck
I am thinking though that I might make an rtv mold of a large section of brickwork, then i could just cast it up and cut the pieces to fit. would save a lot of fitting and carving work.
hopefully pics soon though, still on a borrowed netbook until i can get my laptop replaced on payday