Ross,
You already listed the rust issue, but pay special attention to the very rear tailgate area for rust, both on top, or underneath. And if there is floor mats, or any applied coverage on the driver / passenger kick areas (where your feet would rest) peel it back if you can and check there for flaking rust buildup. This is the most common area I have seen for soft spots.
Other than that, original axles, original motor, brakes (disk vs drum) V8 swap all might be important to look over and see who has done the work, and how well they have done it. (a bad V8 swap can be a nightmare)
If it is running pay attention to steering wobble or lean in one knuckle to indicate worn parts....but if you are purchasing this to be a trail rig / rebuild, all these items are redundant anyhow and need to be replaced / rebuilt eventually.
Check out how easy the trasfer case shifts into low / 4x4, and if there is any audible whine from the hubs / case upon shifting.
As far as cost, depending on condition is the rule of thumb.
I might be available on sunday to check it out with you depending if you want some company.....(I promise to not make an offer as well

)
Josh