Its crusty, that's for sure. But savable. If you use a good product like Krown or FF, you can save the body and chassis. But like you said, you need to replace every line in that truck. I think you are in for more than a few thousand to really go through it thoroughly. Trans, drive shafts, might as well do the frontend, u joints, and everything else. Basically build a new truck and keep the frame and engine. Don't plan on selling at that point, you'll never re-coup your investment.
My 2003 F450 was just a little better when I bought it. While it never plowed snow, it worked near the coast. I'm battling the same stuff, but the first thing I did was buy time by coating the truck very well with FF. But I don't depend on it and I love working on it.
I'm starting to lean to the side of selling and starting new. But personally, I 'd still go brand new.