I take it the 420SEL in question is a 1986-1991 W126 version. Forget the mileage. That car is at least 13 years old. The M116 engine has plastic timing chain guides that wear with AGE and mileage. Ignore the odometer and get into top end a little and check the plastic guides. If discoloured, replace the chain, the tensioner, and the guides. Disaster has befallen many M116 owners that did not pay close attention to this known trouble spot. The guides become brittle and break off. They jam the chain and often ruin at least one bank, potentially both.
Price out an M116 rebuild versus chain and guide replacement. Pay a little now or a lot later...
1998 C230 "Black Betty"