Drew, it sounds like the 1989 300E you bought has been neglected. I have the following recommendations:
1) Fix the odometer so you can track mileage.
2) Replace the coolant expansion tank.
3) Replace the thermostat.
4) Recommend changing all fluids and filters: oil/oil filter, transmission fluid/filter, coolant (50/50 mix), brake fluid and differential fluid ... unless you have records showing that they have been done recently.
5) Replace the viscous fan clutch if the fan is not operating properly.
6) Repair the auxiliary fans.
7) Replace the distributor cap (Bosch), distributor rotor (Bosch), distributor O-ring seal, ignition wires (Beru) and spark plugs (Bosch). Use dielectric grease inside the ignition wire boots.

Unless you are an experienced DIY'er you probably should have a reputable independent MB-trained tech work on your car. Expect an appreciable repair bill. MB automobiles are a pleasure to drive, but they require routine maintenance.

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Fred Hoelzle