Thanks JMichalc for your reaction.
I agree on you first comment, a trade-off must be considered between fast, cheap, less detailed entities and more time consuming and more detailed models. I'm now setting up a comparison. The problem now is that until now I had the impression that the fast-and-easy-way of building entities using the WED seems to be the least efficient way in terms of system resources...
Am I correct in this assumption? I hope someone can convince me that I'm not!

With respect to the second part of your answer:
Is it correct that textures are only loaded once into video memory?
My students maintain that I should reduce texture size and number of textures.

At this moment I use for every house one specific texture for the streetfacing walls, the other walls, roofs etc use standard textures. Adding all texture sizes and rounding them upwards, results in a total of 200 Mb for textures. Is this an excessive amount?
The textures for the streetfacing walls are bmp's, mostly 512x512px and 300 dpi. I selected this combination by comparing different combinations of size en resolution for best rendering.

Any comments on these numbers are welcome!