I'm not really into cars, but it needs to have functional doors or at least fully modeled doors that I can rig myself if necessary, and a usable interior. If detail is lacking it needs to be cheap, because I don't like to spend much on things that are only useful as background clutter.

Here's an example, this is the only car model I've bought so far:

http://www.daz3d.com/i/shop/itemdetails/?item=11260

I agree with BB though, ordinary contemporary vehicles would be nice. I have Stonemason's Urban Sprawl 2, and very few cars to populate such a scene.