At that time of day, you could walk straight through immigration, or you could be waiting 45 minutes (it never takes longer than that *cough*). It all depends on what flights get to the immigration area before yours, its a complete lottery. I (british) have gotten through immigration in 2 minutes flat as there has been no line, but my ex (us citizen) has had to wait 20-30 minutes because of the long US citizen line.

Customs shouldn't take too long unless there are a number of suspicious people in front of you, or you look particularly dodgy

All of the car rental agencies are as good as each other. None of them are in the terminal, so you'll have to get a shuttle bus. I've used Avis 5/6 times at IAD and have been happy. The bus driver has dropped me off right by my car every time. I just get in and go (have an avis prerred service card).

Steve