Make sure it shifts like it is supposed to if it is an automatic. The shift points are indicated on the speedo with little "dots". If you floor it from a dead stop and hold it down it should shift pretty doggone close to those dots. It is alright if it is not exactly on the points but it should be fairly close. There should be no unusual "clunking" during up/down shifts. Also, don't do this with a cold car or you could break your neck. They shift real hard cold. Let the car get up to operating temp before you try it. Another point - drive the car in stop and go traffic with the a/c going. The temp gauge should NOT go above 100 dC and the oil pressure should NOT go below 1 bar at idle. In addition, the oil pressure gauge should IMMEDIATELY go to 3 bar and stay there as soon as you come off of idle. Tell the perspective seller you want to start the car up for the first time that day when you go to test it. Starting a cold diesel will tell you alot about the condition of the engine. If it starts right up and idles fine you will be alright. If it is hard to keep running cold pass it by.