It's my understanding that the engine and transmission on your car are the same as those in my '89 300E. If you'll look around elsewhere on this site, you'll find that many people consider this combination to be extremely reliable. I've found mine to be bullet-proof!
The slipping transmission you mention concerns me. At 130K miles, this thing shouldn't slip at all. My car has 155K miles on it and is solid as a rock. I bought it with 94K miles on it 3 years ago but I've very regularly had my car serviced by an independent MBZ shop (owner trained at the factory in Stuttgart - yes, he IS German). They follow the prescribed maintenance schedule to the letter. Every 30K miles they service the transmission (drain, replace filter, new fluid) and it's solid. I commute almost 70 miles each day and over an 1800 ft. pass - transmission gets its' share of work.
I think you'll love the car and find that the drivetrain is solid and reliable. Good luck and many happy miles to you in your 'new' car!!