I think that the 603 diesel can be a very serviceable engine on ONE major condition. That is to make sure the trap oxidizer modification was done. They remove the trap and in it's place (well, under the car) they put in a catalytic converter. As some of you may know, those trap oxidizers were known for coming apart and sending bits and pieces of themselves into the turbo, therefore burning up the turbo and in some cases, even the engine. Our neighbors have a 1987 300TD with 137,000 miles on it. They have owned it since new and I have always serviced it. When MB had the trap recalls, I had them immediately go and have the modification done. Fortunately, they have never had any type of major problems with the car, even when the trap was still in it. I think that if you find a 300D, TD, or SD with reasonably low mileage that has had the trap removed, it will make an extremely nice car. The 603 is much quieter and smoother than the 616 and 617 motors, and turns about the same economy give or take a couple of miles. So just make sure it has had proper servicing carried out, and I think you will be extremely satisfied, and not to mention surprised at how gas-like this motor is!
'67 250SE Cabriolet
'93 300E 2.8