At this point, the OM617.95 engine in the '81-'85 300 SD (W126) and '82-'85 300 D (W123) is, without a doubt, the most reliable MB diesel EVER. The '86-'87 300 SDL (W126) is a close second.
I've been told that the transmission mated to the engine/drivetrain in the 300 SDL is perhaps the best combination MB put together in terms of longevity, smoothness, and performance. My $0.02.
