Mercedes had a hardness issue at one time. It did not impact the 123 diesels to the best of my knowledge. For some reason years ago the amount of hardness deteriorated towards the front end of the cams.

Other than checking the Cam bearing clearance they should be good to go usually. Unless perhaps the engine has a massive amount of miles on it.

Just remember to number the cam followers. Can wear has not been a real issue with these engines. Even at fairly high accumulated milage. A visual inspection of the lobes journals and followers should be enough in the vast majority of cases.