Well, I'm going to have to repeat what others have said, you're 17. It is so hard to convey what that means because we know it's frustrating to hear that we know where your mind is at, but it's all we can say because we've truly lived through it already.
You have made the goal of moving away when you turn 18. Goals are good, and they sometimes happen, but until you are literally moved out, it's not set in stone. Just like there is no guarantee at your age that you'll be able to afford a horse in a year, or that you'll still be with your boyfriend, or that you'll even be the same person you are now. The next few years will change more than you realize, that's the reality of it.
With that schpeel out of the way, I agree that place is horrid. I am shocked that you chose to subject a horse to those conditions previously, and am surprised you'd even consider keeping another there. It doesn't MATTER that the foal is already there - if you take ownership, he is there because you chose to keep him there, with this thin promise of "one day, I'll move him".
If you cannot move him the second the bill of sale is in your hand, you are not offering him anything better. You point out all these people whose methods you don't approve of, but let me be clear - NOBODY who voluntarily keeps a horse in a place like that has a CLUE what they are doing. Period. Not the few boarders you "approve of", not you as long as you are even considering it, NOBODY. If this trainer you've picked out for this foal is AT ALL affiliated with this place, they are not a trainer you should be hiring. If they really have nothing to do with the place, then you should be spending your time at THEIR facility, not this one.
As for PETA, don't let them fool you about the "power" they have. They are not the animal police, they have no more authority here than you. If there are rules that should be followed, you need to be talking to who made the rules - if it's public property enforced by local bylaws, that's where you need to go. If it's privately owned, you need to talk to the owners. If that fails, then talk to LOCAL animal control, LOCAL media, if you really want to make a difference to these horses. That barn has a lot more problems than a few bad people, it needs to be shut down and torn down, and it's going to take a lot more than a 17 year old with a goal to rebuild.
I don't doubt that your heart is in the right place, and I don't judge you for sticking around there if it's the only barn nearby - I tolerated a lot of dumps just to be around horses. However if you really want to make a difference, work with the right people to get that barn closed down and the older horses especially to a home with good care or a peaceful end. If -as is too often the case- nothing can be done and the people with authority won't budge, you need to walk away. Move away like you plan and find an infinitely better place to work with horses, whether it is to learn what conditions are actually acceptable or whether it is to help horses that you can actually help. If these things don't matter to you then it is JUST about having a foal right now, and you need to walk away. There will be others, even ones you feel that connection with.
Don't even think about keeping a horse with your name as owner in that place for any length of time.