I would expect that a plugged heater core would have essentially no or very minimal affect on creating a high engine temperature. On the other hand, a stuck thermostat or dying water pump will provide high engine temperatures.

Get a Pre-Purchase Inspection. In this case, I'd have the PPI do a compression check of the engine. Even then I'd be VERY suspect of an engine that has been run to the top of the temperature gauge numerous times. What if the gauge calibration is off a bit? Maybe it did overheat several times but the driver didn't see it??

Keep in mind that there is nothing more expensive than a cheap Mercedes. This one could be a simple, inexpensive repair, or have a damaged engine and you have a really nice parts car. Do the PPI, even on a $500 car.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered