The true, "bottom line" number is COST PER MILE. Your 80's Benz likely will have a low cost per mile compared to a new car ! Most people don't get this !
A Mercedes Diesel owner is, almost without fail, the cheapest son-of-a-bitch on the planet. We buy these things and run them forever because they are - at the end of the day - the lowest monetary outlay for transportation possible.