Assuming that the MB dealer is correct about the engine numbers - in other words - you do have a different engine than the one the car came from Germany with, the so called "original" owner is a liar.
How could he/she not know that the car received a new motor along the way?
A lawyer isn't going to help you here because the monetary damages are small in their eyes. You'll spend forever in small claims court battling this issue.
The bottom line is this. You bought a 14 yr. old car with a tampered odometer reading. Someone rolled back the clock and no one really knows the history except the "original" owner who thus far has sold you nothing but BS.
All 14 yr. old cars need work. What you describe does not sound unusual.
Fix the car and move on.