I had an 81 240D towed into the dealer about 14 years ago. The car got towed because the timing chain snapped. I knew someone who needed a new car. We bought the car. We did a head rebuild, timing chain, and all associated parts. The bottom end was fine. Was driven for another 8 or so years before it was sold for a W124 Diesel. I was just really lucky when I took it apart and it was ok. I'd recommend a new engine at this point because you probably won't get that lucky. The 617 is a dime a dozen in the yards. If the paint, suspension, interior and AC plus everything else is ok I would say buy it. W123 and W126s are difficult to find in ok shape at best and great shape is difficult.