The car spent most of it's life in areas that salt the roads; Open the doors and look at the bottom of the door where the weather seals attach. If it is clean with little to no rust it will tell you a lot how the car was taken care of.
I would look at the plastic cover under the engine as well. Is it dry or covered with oil?
Take it for a drive and then stop in a parking lot and use a scan tool (2003 SLK 320 R170) to see if any DTCs are present before returning it.