I have not owned a W124 but I have owned 8 Mercedes. With that said. I would suspect the mechanic. I had the 82 300TD tranny rebuilt in 1990 and have had no problems with it since. I installed a new rear seal in the 240D and again no problems.
If the car is pristene then replacing the tranny with a rebuilt is a good idea. Keep the car going.
BUT
I would first have another mechanic replace the seals. Obviously the current mechanic is not doing something right.
How long ago were the seals replaced? within the year? I would demand new seals from the mechanic. More than a year, find a new mechanic.
I had the dealer do the rebuild and again, done in 1990 and going strong.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car