I am by no means knowledgeable about your model car, but I would advise you not to panic or worry because you will get a lot of help here and we will make sure that your car will run properly. Maybe the turbo boost waste gate is stuck open...happened to my Ford truck turbo diesel. Also, I was under the impression that the 3.5 engine was the "rod bender", not your 3.0.
If the PO knew about this problem and didn't tell you about it, he needs a call from you.
Here's a note from the Good MB Shop section if you are anywhere near Richland, MS
Richland, MS