I just use some fR materials that I have made, converted, or found on their forum. There is a great library that someone made for free on there. Final Shaders looks good for diversity, but just for architecture I didn't see anything that really helped. Plus, the more complex the materials can exponentially increase render times if you aren't careful.

Dosch - is touch and go, I've had great models by them and some just horrible - too bad to use at all.

123d cars are pretty nice. www.cgdatabank.com has the best cost/polygon that I've seen. Very accurate, only about $10-15 each (most of the others are about $150+ each), only problem is their new site is not in English. I tried emailing them, but I have no idea where I sent it!

Just look over CGArchitect and Evermotion for resources and buy what you need when you can. You can probably find enough to get going online, for free (www.3dcafe.com and others).