That's pretty high mileage for a W140...and the price actually seems slightly too high. Unless it's in absolutely perfect condition...which I kinda doubt at 177k miles. It will definitely cost a lot more to keep going than the 300SD.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles