I for one feel a 91 300D will go a long time (or at least I hope mine does
).
Mine was not well maintained but after some work it's a solid car that I recently drove from PA to Montreal two times. You can read a bit about my time with this car, repairs, mods, etc at my Benz-Site (check signature).
As for the transmission, I don't understand the comments in this thread. Personally it's my belief that if you change the fluid and filter regularly it should last. This car had tranny problems when I bought it, but after multiple fluid and filer changes it works perfect.
Most people are religious about oil changes but forget the tranny and diff.