I'm not sure there is such as thing as a "prestigious" Benz dealer--they'll all steal your money
Just because a T-stat fixed your problem, doesn't mean the old radiator was good. I would bet good money as part of the $600, they would have included a new T-stat. Have you ever had to replace the head gasket on this car? A bad head gasket is one reason the radiator can gum-up.
Glad your Benz is back...
-GH