get dealer quotes for fixing the obvious problems you list. Then look up the car in KBB. Split the diff on retail and trade in values, and subtract the repair estimates. That is your offer amount, assuming that is ALL that is wrong with the car.
Just because Drs owned the cars does NOT mean they were cared for. It may be MORE neglected than 'normal'. It may not be run or beat to death, but it may not be mechanically pristine either. Just because they have money does not mean they spend it.