If a W126 diesel has a leaking transmission (slightly), what is the most likely culprit? Seals or simply a gasket? I have owned two Mercedes (1984 190D and a 1985 380SE) and neither had transmission leaks.

I am looking at a 1987 300SDL (301K miles). Transmission shifts good (says the seller), but it has a slight leak he says. The car is out of state, but local to some relatives that live there and I was going to get my uncle to check it out for me, but if its going to be a major problem, I am going to pass. The guy that owns this car said that he has owned 4 different Mercedes diesels and all had transmission leaks.

Thanks.