All good ideas. I would add scratch build. As has been said Matakishi's site shows how using
mainly cork tiles, but also consider 'blue foam'. It is light & very easy to detail, you can draw
bricks onto it using pen. It will sand easily, which is nice if you don't want your buildings to
have that 'Ironed corners' look that MDF often has. You can pick away at it with your finger
nails, for that 'crumbling edge' look to make your buildings look dilapidated. It will take filler,
so if you don't fancy drawing bricks all over it, simply draw a few 'patches' of bricks & 'plaster
the rest. Windows & doors can quickly be done by embossing (push into the foam) with...
just about anything, point of your pliers, screwdriver(s), matchsticks etc.
Matchsticks also good for those half timbered buildings. Roof tiles? Cerial packets & a cheap
set of childrens' craft scissors, the ones with the different cutting edges. I use mine for that,
and there is a more industrial looking edged one that works fine for Sci-Fi doors.