Cool looking project, but without having access to your own 3d printer I do not think it would be very practical to simply buy the STL files.
If you used these STL files to have them made through a 3d printing service, such as Shapeways, I'm guessing one building would cost at least $50 and maybe even twice that - for 28mm.
If I was going to print these pieces as a personal project, the material for one building would probably cost under $20.
A good home filament printer, such as an Ultimaker 2, would probably work fairly well. Printing this type of pieces on an SLA, such as a Form 2, might be tricky but it would look better.
Honestly, as cool as this project looks, I personally feel like there are better options for this sort of terrain.