What is value of 1985 Mercedes 300D Turbo Diesel?

The answer is that it varies widely. I paid $1200 for a 1984 that ran well, with a good body, but it had some issues, like some leaks into the passenger compartment when it rains, needed a good deal of suspension work, etc. If it's been restored, it could well be worth over $10,000. So you need to assess the condition.