From my own experience, fixing a rusty old truck will nickel and dime you to death. Fixing rusty panels has doomed my project to corner ir the shed purgatory. Yes. I do have buyer's remorse. If I had to do it all over again, I would buy a running and driving truck and enjoy it, regardless of the condition.
In your situation, a chassis and new cab might be the way to go. After you get over the sticker shock, it may save some money and time in the long run.