It sounds like you really want the '87 300d. Don't let the 617 guys talk you out of it. The '87 300d with the 603 3.0L motor is not high maintainance or troublesome and gets 30+mpg on the highway. You can run extended oil drains, not have to adjust valves, etc... The only problems I can think of is head cracking and maybe lifter tapping at times, other than that, Its a diesel Mercedes like the rest of them except faster, smoother, and quieter than previous models without the crazy electronics of the newer diesels. W124 parts are also comparable to w123 parts in cost and the w124 is actually pretty nice to work on, nice layout, good engineering, ample work space....

Under $3,000 for a fine running 87 300d with under 200,000 miles is a great deal. Go for it! These are great cars that deserve proper care and more time on the road!