I look forward to the finished project, Jochen. The changes you made to the fire engine were quite substantial in their overall effect and, as I've just reminded myself, improve the appearance of the fire engine greatly.
When I made a low relief set of buildings out of MDF, I used a commercially produced brick paper for the walls. The actual scale of the paper was 1:16 as the 1:24 scale paper, although about Playmobil scale, didn't look right. I also used a "worn" version rather than a brand new looking brick finish.