Import Motors. I got to them from comments here suggesting that even if they aren't cheap they are honest and reliable. They seem knowledgable also, but I'll advise further after I get the car back having fixed the six problems that developed after I bought it.
I've also had references to All Mercedes and to Fritz & Peters, but went with Import as it has been around a long time.
1987 560SL 109,000 miles
2001 E320 103,000 miles
2007 E350 32,000 miles