How to spawn random buildings

Make the different building elements in some modeling software.

Instantiate them randomly with scripts.

Or build the city manually, and accept that it is always the same.

The particular functions you'll want to investigate are

Instantiate( Resources.Load("AResource") )

Random.value

I agree with Atnas. Your best bet would be creating the buildings and roofs as seperate models in a modeling program like 3ds Max (which you can get a free 30-day trial of), importing them into your Unity project, then using scripts to randomly select and spawn said models.

A very simple solution might be to use a jittered grid for spawning buildings, especially if they are roughly the same size. See this link for information:

http://umbcgaim.wordpress.com/2010/06/17/distributing-stuff/

And here is an interesting paper on random point generation (which could be helpful for spawning buildings too):

http://jenny.cartography.ch/pdf/2010_Jenny_etal_PointPatternSynthesis.pdf

And this could be helpful for generating non-regular grids:

http://gram.cs.mcgill.ca/papers/rudzicz-08-iterated.pdf

Here is some more information on procedural city generation which might find helpful. (It might be overkill for what you are trying, but the ideas are interesting and you could probably simplify or adapt it for your needs):

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.91.8713&rep=rep1&type=pdf

http://ccl.northwestern.edu/papers/ProceduralCityMod.pdf

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.92.5961&rep=rep1&type=pdf

Check out my asset BuildR for generating Buildings at runtime or in the editor.