Since you might not have a large disposible income at this stage in your life I would not buy a 1987 300d. For starters it is somewhat more trouble prone than the earlier mercedes diesels. If you want true fuel economy plus you are willing to work on your car to some extent if a diesel I would consider either volkswagon or perhaps a toyota or honda gas car. Expensive to maintain even doing the work yourself cars should be avoided like the plage unless it is partially like a hobby and you do have the resources. You also really cannot put milage back into a car at reasonable cost even if possible and 300k is too much even if it is the original milage. I hate to see anyone put themselves into a box. Now if the car is say 500.00 you should be able to walk away from it if you have to at one stage or keep it for when you do have more disposable income when it breaks. Just my opinion.