The most notorious Mercedes for eating up transmissions are the M-100s (600, 6.3 and 6.9). My 6.9 is on it's second tranny and it has begun to slip in first gear and reverse, so it looks like #3 will be in order shortly. I don't drive it hard at all nor did the previous owner, these boxes just handle so much torque that they are very likely to fail, and most do. However, my first 6.9 had 203,000 miles on it on the original tranny, so some must just last longer than others. My '83 300D has 292,000 miles on it and shifts nicely, although it does shift hard on the 1-2 upshift. That will be cured by an adjustment to the vacuum modulator.