Hard to go wrong at $6500. I would wear trashed clothes and take some time crawling around underneath. He should be willing to jack it up a little for you. Bring a good flashlight too. You want to really examine the frame rails, cross members, rocker panels as well as the floor and trunk. You will always find a little rust but what you're looking to avoid is where it is so heavy that the metal swells out with scaly rust and crumbles when you poke at it with a screwdriver.
The first thing I check on any car...I examine the inner fender panels, radiator support, and front frame rails by the bumper brackets for wrinkles and warps as well as the gaps between the door and front fender. Those are dead give aways for big front end collisions. Even a car that was pulled will have warpage in these areas that you can see. That's not to say you shouldn't buy it but you need to know what you're in for.
Most of these cars have rust at the bottom of the front and rear windows in all four corners. Some guys put sealer in there which doesn't work but clues you in that water leaks there. If you will be storing the car outside, not a bad idea to pull the front and rear glass, have a good shop weld up the holes, and properly seal the glass back in to avoid future water getting in and creating more issues for you later.
Good luck!
