I'm not sure of the failure mode for engine mounts, but it's worth a check. I know the MB engineers designed cooling ducts from the front of the car specifically to bring cool air to the mounts during driving (tke a look in your car - it's very strange). I guess that the heat of the M119 engine can deteriorate these mounts.
Does it seem like your car is missing or knocking? Or is it just a general roughness?
Mileage of the car would help diagnosis too.
Troy 1995 E420