I own both an '85 300D and a '90 300SE and had an '84 300SD. My opinion is you should find one that's well cared for with at least 175K on it. That may sound contradictory to what everyone has been taught, low miles being better.
However, my experience has been that the higher mile cars, if taken care of, have had all the major $$$ stuff replaced and are ready to go another 100K miles. Further, you can pick one up cheap because most people are afraid of any car with more than 100K miles. My big three to look for are...Tranny, A/C and paint. Each one of those can run $2K plus.
For example, I bought a really nice 300D with 125K on it about three years ago. I've replaced alot of stuff that probably adds up to $4-5K (not bad really over 3 years). However, the two S class cars bought with over 200K, I spent less than $500.00 per year on repairs.
All in all, I think a 300SD is a sure bet. They ride great, get 25MPG and your friends will think you must have gotten a raise. Just keep $1000 in the bank in case a big repair crops up and you'll be OK.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)