These issues may have been relevant if you had been W124-shopping eight years ago.

By this time, most of the classic issues (for any of those model years) have been rectified by the current or previous owners, as the typical mileage on these cars have reached the point where the problems have surfaced within that timeframe.

As with my car and many others that own one on this forum, you will probably experience wear-and-tear issues that will not fall under the items already posted by suginami and possibly others later on this thread.

Unless you manage to obtain a nice example by a fastiduous PO, expect to do some suspension tune-ups, especially bushings on the high-mileage examples.

You may have to deal with cooling systems components, an alternator or maybe a relay or two, and perhaps an oil pressure sensor, or coolant level sensor.

In other words...don't sweat the hearsay...these cars are probably beyond that.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car