Well, since I now know where the models came from (I won't say any names... but my God... Steve, you are terrible), we knew where the problems came from, yes.
The textures were roughly 12MB big, and heavily oversized (1024 upwards for a simple building? COME ON?!). In addition, his meshes weren't attached to each other, resulting into at least, in a specific case, to 20+ drawcalls, each having 12MB of textures.
My final advice to the models that he bought from that place:
-> Optimize the meshes heavily, and remove those polys that are not used/not seen
-> Optimize the textures (12MB PER TEXTURE?!)
-> When modelling a complete city, and if you want to export it as a whole, while it uses only a few textures, my advice is that you do NOT use "Group" in 3DS MAX, but that you use "Attach" ("Attach List" for the lazy people like me and select all, while having the base object selected), which results into significantly less drawcalls.
Modelling is more complex than programming on some parts of it!