I remember that car on bring-a-trailer, congrats! And like Sixto said, start with all the fluids, filters and do a coolant flush and replace the green stuff with the right Mercedes approved coolant.
There is so much information out here and on the web about the W126 in general or the M103 engine. I've tackled a lot of things myself on my 1990 300SE, including a head gasket and front suspension with the help here or the information I found on the web.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).