I am in process of closing the deal on an '83 240D, no rust, clean interior, 117K miles on odometer, automatic. Did a preliminary mileage check over about 60 miles, seem to be getting about 23mpg on the "Interstate" (even in Hawaii). Could be procedural, maybe sample too small, but I am curious what sort of mileage I should (or could) expect with this otherwise gorgeous 240D.

That may be about right in hilly territory. Flat running could give a llittle more. Maybe about 26-27 mpg tops. As you mentioned 60 miles is almost impossible to form an accurate picture.