Don't feel like we're trying to gang up against you or anything - I think people are posting in good will that things might not be as bad as you have been told by your mechanics.
I can tell you for sure that if you took the car to any Mercedes dealership in the DC Metro area, their quote by default will be "worst case". I have a 1987 300D that was diagnosed by an unmentionable DC area dealer for "possible cracked head" because it ran rough at startup idle and smoked a little during warmup. It was a bad glow plug. $10 to fix.
For one example, I put new rear stabilizer links on my '82 300D for under $10. EBay. They just bolt on, and the old ones were really worn out and making a terrible CLUNK when running over any kind of bump. I wish I could get work replacing those for $93 a set!
I'll bet a ton of people that have owned the old MB diesels never read their owner's manuals and never had any idea that the valves had to be manually adjusted. In fact I know one person who had never had their valves adjusted or glow plugs replaced. Getting that done was like getting a new car.
So, take your mechanic's evaluation with a grain of salt - they're in the business to find anything possible to repair on your car. And let me tell you, those bad $10 links in the back sound like about $500 of repairs easily.
Ken300D