Judging by what you describe, there is something more than just "being out of spec" wrong with your tranny. That being the case visiting, a dealer, reputable independent, or german tranny specialist would be a good idea. That M119.974 engine is really a torquey SOB. Given many years and miles of aggressive driving it can surely be hard on a transmission. Especially if the previous owner did not follow MB maintenance intervals precisely.
Let us know how it turns out...Lee