Back corners rust, especially around the box that covers the gas filler neck.
The front floor seam usually rusts out and the front floor pans.
Watch for frame damage. I found a few that the vehicle has hit a ditch and they buckle over the wheel wells. You can see this if the doors line up or rub against the back of the door post and top of the wheelwell. I have a nice parts van. It would of been worth restoring for lack of rust. But the guy hit the ditch and bent the frame.
A friend was going to buy a van. Good thing I went with him. I saw the that it was bent right away. Due to this he got the van for way below asking price and uses it for parts for his other two vans.