There are a lot of Web sites that carry lots of building add ons doors, windows and roof items as well.
- Ground Zero Games
- The Scene
The "Aldi's ones I'm still thing of a way to make them open up so as to play through them as well as around them.
You could also check out HO Scale trains as well to dress up you buildings of scratch build items as well.
A good base for mounting your buildings on would be thin sheets of Plexiglass, it takes glues well thin and light weight resist warping and the corners will not bend.
I would also suggest filling the larger containers with foam or something to stiffen them up the roof's will flex a bit if not and could pop your details off during gaming and such.
As you can see I have quite the collection going on here and I have multiples of each one posted, I can fill a rather large table.
So now get out there eat well and play well.