The 1987 124 cars as they are known are pretty trouble free. Unfortunately they are not quite as solid as the 123 series IMHO but make up for it in refinement and power. There are two areas of potential problems that come to mind that you should be aware of.

1. vacuum pumps. These have been known to let go without warning and spit parts into the cam chain with catastrophic results. If you are going to keep the car check to see if the pump has been replaced with a later version. There are lots of threads here on the topic but IIRC the newer pumps have rivits on the front cover and the original do not. There are threads here with photos of both types.

2. cracked heads. This is usually associated with overheating but I'm not sure any of us can say that is the only cause. If your head has a part number that begins with 14 stamped on the side under the intake runners you have an original head. Lots of cars, including mine, are still running around with the original head but many others have had problems. Mercedes has made a couple of redesigns and if your head develops cracks you'll need to replace it, it can't be fixed. You'll know if the problem is developing if the cooling system remains pressurized well after the engine is completely cool. Again, there are extensive threads here about the heads on these 603 engines.

All in all the 124 is a great car and the one I prefer for long highway trips. Fix it up and keep it, you'll very probably love it.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold