Can't speak about the E300D cause I have no experience with one. I have a 1990 350SDL. It has 155,000 miles. I am 2nd owner and have owned it for only the past 6,000 miles. Impressions are that it is built like a bank vault. I have averaged about 28.5 MPG. My daily trip is 40miles of mostly highway. Has lots of power and very smooth. IP pump was bad when I bought the car. I would run a compression test, bit of a bother, but that should help in checking engine health. Also oil usage is a good indicator. If the 350 that you are considering is a good one, you will have a very satisfying car to drive. I would bring either car to a qualified technician that has lots of experience for an opinion.
Regards,
Andre