Marlene, my '83 300TD, has 226,000 miles and is really slow to engage reverse but shifts upwards solidly and exhibits no sign of slipping.
She slipped, flared badly and blew moderate throttle shifts when I bought her but some tinkering with the Bowden cable and fixing vacuum issues corrected all but the slow to reverse problem.
Maybe you can extend the life of your trans by performing the adjustments here:
http://www.wagoneers.com/DieselBenz/Trans-ADJUSTMENT.html. I used this procedure but was able to achieve good shifting without having to go into the hydraulic portion of the procedure.
I also think you really should go after that fluid in the converter to eliminate what sounds like some seriously toasted trans fluid.
To access the torque converter drain, rotate the engine the correct direction at the harmonic balancer with a 27mm socket/ratchet until the coverter drain is accessible through the opening in the bottom of the transmission housing.
R Leo