Is your mechanic an indy Mercedes mech or a 'do any car' mech??

If the former, he should be knowlegeable enough to warrant the hourly rate.

Sometimes a hard-stiff top radiator hose only after constant running for over 35 miles can indicate a slight head gasket leak.
There may be other indicators like the sniffer test mentioned in prior response.

You, not a mechanic, suggested something valid. A comp. test.
How much is the test? How much is the labor on the rad swap?
What is low retail on the radiator?
Pay the difference and keep your old radiator and sell it for cheap.

take care.

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD