You most likely have a leaking secondary pump o-ring. This can be changed without dropping the transmission.
I've had the same problem with my 88 300CE. The car has 125k (Guam) miles and the dried up o-ring has been the only problem so far.
The only other problem that I've seen on these transmission is the primary pump bushing wear. Other than that they are pretty bulletproof and should last a very long time with regular fluid, filter service.
Unless you are having other problems (slippage) or front seal leaks don't drop the transmission.
I suggest you find another mechanic that has more experience with Mercedes.