My first question is how much experience do you have with horses?
Taking on a rescue horse is vastly different to a normal horse. Rescue horses come from bad situations, and often some of their problems aren't curable. I feel that sometimes people take on rescues as a pity case, but are often left with a horse they can't handle and the horse gets worse.
At your age, its quite a big decision to have a rescue horse and I can tell you by the time you hit 16 school will come down on you hard. You'll be so caught up in work, your social life etc that you won't have much time to spend with your horse.