Posting some pictures would help I guess.
About the polycount, I think some levels have some sort of buses already, I would go and see how many sides their wheels have, how curved the bodies are, which details are modeled and which ones are painted on/normal mapped etc. Then try to match those polygon densities. You should also be able to check the models in Valve's model viewer(included in Source SDK)
Just in case you missed this site: