Without a long story involved...
Your mechanic sounds like my former mechanic. Nice guy, doing his job the best he knows how, not trying to rook you, but doesn't know these cars inside and out. Maybe take other cars to him, but for your M-B, find an Indy who specializes in M-B, particularly our tried and true diesels. Pay your guy what you owe him, but at least ask for any discount he can offer since it ended up not being the radiator as you suggested. Maybe he'll do something. Maybe not. If you like him for work on other cars, stay on his good side by saying it was worth a try and pay what he says. Then, try not to take your M-B back there. Just don't tell him that.

Here's my suggestion..keep a pile of business cards in your car, or print up small flyers..
Anytime you see an old M-B parked, go out of your way to get to it. Leave a note admiring their car,saying you have an old diesel MB and are looking for a good mechanic who knows these well, that you would appreciate a name and phone number or address.

Worked for me. I ended up with a fellow also suggested on this forum. I couldn't be happier.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."