Check the pinch weld at the roof line above the windshield. That is a potential rust area. Cab corners and seatbelt bolt area on the floor is an area where I had one rust out as well.
They are tough, no frills trucks - and I enjoyed beating the snot out of several of them over the years. Specifically with a Cummins version, check and see if it will do 70 on the highway - the 3 speed auto versions coupled with typical truck rear gears can make for a truck that can pull a house down, but can't go much past 60 mph.