Ian, find out what year it is then we can offer a reasonable guesstimate at what it might be worth. However, without some good photos it's gonna be tough.
Example: Rust ain't gonna creep up in just one place. If you have some fairly severe cancer going on on one body panel you'd better look real close at the rest of the rig. Check out the tub and floorboards especially and not just a visual but take a screwdriver, knife, awl, or something like this and give the metal some pretty good taps. If you punch a hole, well... Also, pay close attention to the frame. Get a flashlight and try to look into the frame rails as well. Look for heavy corrosion and/or any kind of cracks.
Obviously, there is a whole lot of other stuff to look at when considering a vehicle purchase but unless you are a good welder and have the time and desire to do a lot of patch work, the rust can be some of the most troublesome.