Step one: get done with basic and AIT, or whatever your branch calls it, and get to your first duty station.
Step two: do a lot of research, and not barracks hot rodder research. 98% of that is bs.
Step three: unless your goal is to build a dyno hero, figure out what you really want your car to DO. 800 hp is only useful for barracks hot rodding (see step 2). Do you want a street/strip car? Autocrosser? Open track/ HPDE car? Or just a stoplight racer? Once you define that, build for a goal. You'll very likely discover that a stock 5.0, or mild bolt-on/ tuned car will give you more than enough power. Unless you're shooting for low 9s, 800 hp is overkill.
Step four: once you define your goal, find out what others have built to meet similar goals. Develop a build plan, then stick with it. Don't get sucked in by shiny ads and marketing promises. Build handling that gets your power to the ground effectively, braking to control it, then build more power if necessary.
Step 5: don't be a typical Joe and sign up for a car that, with payment and insurance will consume 50% of your paycheck before you even start to drive it. Unless you are so addicted to **** that no live human can measure up, you're eventually going to start dating. That costs money, and contrary to all the ads in the rags, very few women are going to crawl into your car with breasts heaving, wearing nothing but a thong, and a "take me, Coyote stud" look, because you have 800hp and $500 a month discretionary income. If you're successful at dating, you'll eventually get married, and she's not going to want to live in a trailer park full of crack heads and child molesters so you can keep your dyno monster, at which time you'll end up selling it (you'll take a huge loss, btw, barely covering your note, and not recouping your down payment, let alone the $20k in mods that you'll need to get to 800hp).
Step six: if you're not ****ed, and are still reading-- start your plan with an 11-12 used 5.0 . There are some out there that haven't been nodded, or hammered on, that will be good foundations for building a sweet car, that will meet step three. You'll have half the money in it than you would have had in your original plan, and should still be able to keep it, and afford a decent apartment with your new bride (until you figure out what a PITA it is to put a car seat in a Mustang).
Yeah, I know I sound like your dad. I'm old enough to be your dad. I am also retired military, and have seen thousands travel the same path I did, and you're about to. You can learn from our mistakes, or recreate them. If you choose the former, you can get an early start on a great life, where you can have much fun, play with cool cars, and have an amazing family... over time. Or you can think you're smarter than all of us who tried to have it all now.
Sorry for the rant. Just giving the advice I wish someone had given me, and the dozens of troops that works for me that I had to help unscrew their finances and lives.
Good luck in Basic!