Did you happen to get pictures of the frames (especially c-channels)?
My truck was a polished turd when I bought it, but I "needed it." Wanted a 60 series since I was a youngster.
Anyway, when rust bubbles started popping up in all the usual places, I just let it go. But, then a couple of years later, I borrowed a welder from a neighbor, learned how to weld (not beautifully, but good enough after grinding). Cut out the rust and put in new sheet metal (from Wolf Steel out of Canada... panels took some work, but they did actually do the trick). Had to source a new tailgate (upper lift) because I could poke a hole through it with a pen... Anyway, with all of that finished, pulled the glass and then Monstalined her in 2-tone. I don't care about resale value because I'm never going to sell it.
I haven't ever added up how much I've spent on parts, supplies, etc. It's a lot. But, I don't regret it at all.
If you could talk the seller down, as others stated above, and if you don't mind going after the rust, and if you have the extra cash (I would pull that engine, clean it up, replace/refresh as necessary, reseal the whole thing and then plop it back in).
Sorry, this message got longer than I intended... also, my final big question is - how is the transmission? Pushing 200k, you might be lucky and run another 100k, or you might smoke it straight away.