Again, several of us have built complete cars.
Despite that, what worked for me will definitely not work for you.
There are plenty of guys that have similar builds that you can take guidance from..I guess if you want a path to follow, sooner or later you'll find it.
From looking at your profile, I see that you're 20. My guess is that based on my list of must haves you don't have much experience in any of the stuff I said you need.
So I'll offer you this advice as my only input.
Unless you already have a car, I'd buy some bodies else's sweat equity. If you truly have 15k to spend, the fox you can buy for that much money couldn't be duplicated for almost twice that if you do it yourself.
I have 15k+ in my car. In addition to that, there is custom labor that would easily be double that much if I had to pay somebody to build it for me,....so,..a car potentially worth 45 k.......that I would struggle to sell for a third of that.
A heavily modified fox mustang is typical of the same poor return on investment,..so bottom line...there's a gold mine out there just waiting to be found, and all you have to do is go find it. It's also the best car to change stuff on if you just have to do that, so in effect, a good place to learn. But not necessary.
If you do have an SN 95 car, and you still plan to modify that yourself, be Leary of the fact that they are not as popular,..and a big investment in your part in one of those cars will do even worse if/when you decide to sell...so think about that.