See how they start, COLD most importantly. I bought my 300cd, and when i looked at it the owner had warmed it up to hide the extremely poor cold start of the car. See how they idle under load too, more important on the gasser, a rough idle can be signs of problems with the injection system. Last thing, check the trans fluid, most often neglected by owners, make sure it is red, not black or orange as they can be signs of high temps or way past change intervals.

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.