Head gaskets etc.
My father's M104 ('93 E320 coupe) at 120,000km needed a wiring harness. It has not needed a head gasket unless the previous owner had it done at under 100,000km. It is now at about 130,000km. Its water pump occasionally leaks a very small amount of coolant.
An M104 ('90 300CE-24 Sportline) with 180,000km I looked at on the weekend had noticable oil in the coolant. Externally the engine was clean but had been detailed for sale!
When we purchased our M103 ('90 300TE) almost 5 years ago it leaked at the rear of the head. It progressively got worse over the next almost 2 years when oil appeared in the coolant at about 145,000km. That's when I pulled the head off and fixed it. 175,000km now and all is well including still dry at the front timing cover.
At the time of purchase my M102 ('90 190E 2.3 Sportline) at 211,000km had a hint of oil in the coolant reservoir. Previous owner claimed to have head gasket done. I flushed the cooling system and have since done a long interstate journey (as well as it being my daily driver). No oil has since appeared in the coolant and it uses no oil. Maybe the previous owner was telling the truth and the coolant reservoir simply had not been cleaned properly following a head gasket failure. The reservoirs are tricky to clean with all those internal baffles.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.