You're gonna find that if you walk into a commercial shop with your idea and literally make them do every step of the work including find the original body, six months later there will be no $20K and half a truck at best. For a first hot rod without having much experience, that's crazy dreaming to go that route...(I can't begin to describe the pitfalls that you and the shop are both going to encounter and make it through in order for such a project to be a success).
There's plenty of guys out there who have built their nice truck, with huge money and time invested in it, and now need to sell at a loss for whatever reason. Take your cash and go shopping for one of those, be flexible with details and try to find a good deal around 15K, then you have a little left to fix issues that come up or re-do something your way. Later you'll start having the experience you need to go building things, or having them built if you choose to make that your thing.
And congrats on your good fortune. Don't screw it up!