 A used Mercedes, who brags about quality, should not need a new cylinder head at 65,000 miles.   Running rough? When they do a compression test, dont be suprised if it is number 2 or number 4 on the passenger side. It will cost you $5000 and you can forget about warranty coverage. The electronics are way out of date. Save yourself money and frustration...do not buy used.  And dont be misled when you get a call from Mercedes "corporate".  It is some hourly employee who gets paid to tell you how special you are but they wont do a thing about their crappy motors.  I will never buy another one. My AUDI Q7 is much better all the way around.