I think the ‘65 F100 in your original post is a great starting point. I don’t think you could duplicate that truck for the money, and it has a real solid drivetrain to sell if you decide to swap it. With the CV frame, you could easily do a modular motor and a budget turbo setup. Put on some deep dish wheels front and rear with a good tire and you’d have a hell of a truck within your $15k budget I think.