Frame corrosion is a deal breaker. Salt from beach drives or winter roads can ruin a vehicle fast. Even fresh water from stream crossings can start rust.
There are lots of posts on this site about under body/frame corrosion, just use search function to find.
Take lots of pics of the frame, body, engine, tranny, diffs, etc by getting under it with your phone. Post them here with questions and you will get lots of opinions.
Parts can be repaired/rebuilt/replaced when worn out-but rust kills 4runners.