I look at it like this. How much time and effort will you spend to repair the rust. And how much more do you need to spend to not replace rusted body parts, and it's always worse than it looks at first inspection. Plus, 15k miles. I'm a skeptic on that one.
Nice truck, you could probably get it for 2k-2.5k. Remember the bucks are going to go down the drain quick on a project. So, two years from now the price of the vehicle won't compare to what you've got into it. Ask me how I know! And mine hardly has any rust, and I still hardly have time to do everything I want. For me bucks are not the big issue(though I like a deal a much a the next guy) it's time. Keeping up on repairs and making upgrades. I need more time.... can I borrow some time?