As long as rust/abuse/an accident don't kill them, they can go on for virtually forever if you replace parts fairly steadily.
My dad's two W126's both have in excess of 300k.....one is at 306k and the other is at 315k (with a new motor though, but the trans is original on both!)
Mine has 280k and has run exactly the same (perfect) since the day I bought it with 207k on it 6 years ago.
MB's are meant to run a looooooong time....even the gassers can readily go 250-300k+ with proper maintenance....my 300E just rolled 139k and the engine runs quiet and strong, like new.
What it all comes down to is what you are willing to repair/spend to keep it on the road.....in my case that willingness has no limits!
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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)