What is the mileage? Are there service records? I would pay to have the car inspected by a independent specialist who knows these cars well. Headgaskets and wiring harness are common failure points on the M104 series engine. Check to see if they have ever been done or budget for their replacement in the future. If the car has ASR(traction control), make sure it is working correctly. ASR can be problematic and costly to make right. No heater core problems, perhaps you are thinking about the A/C evaporator? They are known to fail, but that doesn't mean yours is destined to. However, the evaporator replacement is a very costly and labour intensive repair. A search will bring up more info on what to look for.

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1989 300CE