I just had the timing chain replaced in my 85 300d. My tech suggests to replace the chain @ about 200K. I have 229k (only 229k) now so it was about time. He also said that the 123's don't tend to have any timing chain problems but the chain is much cheaper than a replacement engine. My cost was $370. 4 hours labor to replace the chain & do a valve adjustment. The chain part was $138. $600 is a bit steep for one car.