I know this isn't what you want to hear. I would stay with my coach and keep working on position and then when the time came I'd probably lease a horse that could do the increased heights. The reasons I suggest staying with your current coach and leasing the next horse are these: As you say, within the next 6 - 12 months your posistion will have improved to the point you're ready to move up. The economy is terrible right now and while you 'think' you have a good paying job in Jan, that could change anytime between now and then. Leasing a horse leaves you a little more wiggle room if the horse turns out not to be what you wanted, you can lease more horse than you can afford to buy and if, God forbid, you should lose your job the worst thing that will happen is the owner takes back the leased horse. You're not put in a position that if you lose your job your horse goes hungry. And if at 18, you find that demands of life, school, boyfriend, whatever are such that you no longer care to have a horse, you can just finish the lease and return the horse, not worry about having to sell it.
What I see happening in my mind's eye is this: You stay with your friend and her mom, in 3 months you have stabilized your position and in 6 months you've totally outgrown your leased horse and are ready to move up. You've now burnt your bridge with your coach and she won't have you back though she's got the horses you need and you can't find another coach you like as well with the good horses....leaves you out in the cold with no horse and no coach.