Im not sure whether you'd be able to use loads of cars without pushing the polys up way too far, but it'd be interesting to see what you come up with.
I think the map Tha Brotherhood Cosmic Club by SinisterMinister (cosmic3.bsp) has a selection of vehicles on: an SUV, a sports car and an articulated trailer and cab.
I don't know where we are in the discussion about whether it's good practice to decompile other peoples maps and take out things like that, so I think you should at least look at it to get some pointers.
- Dag