I replaced the transmission in my 89 190e 2.6. I think they are fairly similar cars as far as the engine and tranny go. I found a quailty salvage tranny for $850 with 30k miles. Pull the engine and transmission out at the same time and swap out the tranny. Took me the better part of a weekend to do. I spent one day pulling the engine and tranny and the next day installing it. Fairly straight forward job with a copy of the engine manual. No special tools needed besides a torque wrech and some hex sockets.
However, try changing the fluid first if you have not already done so. My old tranny did the same thing and would usually get better with a fluid and filter change.