This is a real toughie to answer. I have a 300SD which I love, but newer vehicles are starting to look a lot more attractive to me these days. If you must get a Benz, newer 300D's of later body styles (late 80's to early 90's) are starting to show up with quite reasonable prices and these cars are quieter, more powerful etc etc.

There are some pretty decent newer vehicles around now that out perform an old SD in fuel economy and many other aspects as well.

Do you view the car as a hobby or as a means of transport? Suggest you need to think that through carefully.