If anything about the door is not original, you should see it by the sealant on the inside edges of the door.
The different steel skins are sealed (with kit) at the innerside of the door. Nobody can make the seal look as good as mercedes does with the original doors. If one replaces a damaged door with a new, original door after an accident, mercedes delivers a door without the sealant.
If the door looks the same and no traces of possible damage can be found, the car hasn't been in an accident.
I would stick with the suggestion John wrote down and have a look whether all parts are still in good shape (no broken clamps/clips) and have a look whether the tare plate is stick inside the door (if this tare plate is missing, the "thud" sound is gone as well).
If all is alright; don't bother any longer...enjoy your car! (instead of looking for all possible (minor) malfunctions)
with a minor 600+k