I have also been at Z. Doberstein's garage a couple of years ago, however I found him a bit expensive and hard to deal with (he is rather moody). The guy I (and most of my friends with german cars) vouch for is Charlie McNally at McNally's auto in the Finch/Dufferin area in North York. He is very friendly, knows Benzes and Bimmers inside out, gets a lot of business from independent Merc. dealers in the area (there are a lot of them).
He even gives you a loaner (it is not a Merc. but no charge :-))if you are badly in the need of wheels. All he is asking for is to put gas in it when you return it. His prices are pretty reasonable too. He is an Irish fellow with a thick accent which I find endlessly amusing.
I talk to him just hear it.:-)

McNally Auto Service Ltd
(416)398-6799
49 Toro Road
North York ON M3J 2A4