I just bought a 91 LWB and went the through the same process you are going through. Here's what I discovered with regards to rust:

1. Check the front wheel wells, espcially the inside panel that it shares with the cabain area near the top.
2. Front fenders where the mud flaps attach - they collect wet mudd that rusts the metal.
3. Rear mudd flap brackets- ditto- wet mud collects between the rubber and the metal bracket. Brackets are no longer available.
4. Open the tail gate door and check the door sill- some people have reported rust in that area where water collects.
5. Front and rear bumper substructures- just sandblasted mine. Not necessary if you're adding a new bumper.
6. If your car is coming from a winter climate, snow and salt collect inside the rear bumper rubber corners- which are like scoops. You'll want to see how bad it is there, but this can usually be sandblasted away.

Hope that helps