You get your current car (or cars) and fifteen grand to spend on it (them). What do you do?
Do you spend every dime on parts and labor? Or just parts? Do you pour some of it into what you may still owe? Is it fifteen grand worth of race days for you, or maybe it's your car plus the fifteen grand into a new car? There aren't any rules. Spend accordingly.
It took me a while to puzzle it over in my mind, how I'd answer this question. What I came up with is that for any of the money to go to my car, other financial issues would have to be addressed first and so the only way I'd have $15k to play with would mean I'd taken care of the rest - so a portion of a win fall or something.
That being the case, I'd spend about three grand on incidentals. Some of that would go towards the small debt I owe on the Mustang and the rest to a couple expensive tickets I may, or may not, have incurred this year (and the associated points). That'd leave me with about twelve thousand dollars to play with.
I'd toss around the idea of putting a crate motor in, a Coyote 5.0, but I don't think that's the way I'd go. Instead, I'd have the 4.0 fully rebuilt, built and tuned. I'd put about five thousand into the motor and transmission including exhaust and an 8.8 rear end. After spending another three grand or so on suspension and wheels, I'd spend another two on aesthetics; seats, dash, etc. Erring on the side of caution, that's what? A two thousand dollar buffer which I think is smart if only because I'd be getting it done by shop and paying labor.
So, basically, I'd take it to the right shop(s) with a budget of $12K and be like, "Let's do this."