I am leaning towards the radiator being the issue, your description of the fan sounds fairly normal.
I have a good radiator from a 90 300E if you are interested. $80 shipped via fed ex ground.

If you want the clutch fan and radiator hoses as well, $135 shipped.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes