A leakfown test will tell you exactly where the leak is coming from. Listen and it reveals. If it is just a head gasket, I don't personally think it would be wise or financially wise to put another motor in. It's a very easy job for the head gasket. If your engine never got hot it's likely the head isn't cracked or warped. At that point a head gasket and valve job will restore the compression. The bottom end is VERY strong on this car. The OM617 is the cream of the crop and then is the OM606 engines. Theres no reason why the bottom end won't exceed 400k miles.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k