I have now owned my 91 300D 2.5 (100k miles) for a year, having purchased it with reference to this site. I would not hesitate to replace the engine --after all, you know the car's history (for as long as you have owned it anyway). You also know how it has been maintained.
Superb handling, comfort and fuel-efficiency make this (indeed rare) car a perfect balance between the solidity of the earlier 300d's (I have a 79 300SD) and the amenities of later ones. And parts aren't a problem.
Imagine 300,000 more (relatively) trouble-free miles, and take the plunge.
Think about the 38mpg you get on the highway.
Think about how much you'd have to spend to get the kind of motoring you're accustomed to.
Good luck. Simon