Look very carefully for rust. How many miles? Any service records? Drive it an see how the tranny feels, look for any flaring or slipping (it's not unusual to put a tranny in these at 250-300K miles. Are you willing to live with the dash cracks (that would drive me nuts and it's fairly expensive to replace). If the climate control is flakey, you will eventually spend a few $100 replacing the ACC (after you take it apart and try to fix it several times, of course). The engine should sound about like your 240D, no unusual noises. Mostly just common sense.

Do you have a good shop? Take it in for a pre-purchase inspection to make sure there are no major issues.