Take a camera with flash and place the camera facing up, resting on the ground under the body and take:
2 -3 pics per side behind the wheel to bumper, about 5 pics spaced evenly to cover the rockers inward, starting just behind the front wheel to the rear wheel. -3 pics under front and rear bumpers side to side. several close up pics of the T top seals (look for rust staining), pics of gas tank and bottom of engine oil pan and top of engine compartment
The reasons: the rear frame rails rust just behind the rear wheels, the floor pans rust from water leaks especially pass side and the windshield pillars rust from T Top leaks.
A few months ago I was searching for G bodies and I took pictures of the undersides and reviewed them on my PC before making decisions.