First off, cushings is a disease that horses get....
Second off, this sounds like a bad idea overall.
You are buying horses WITH FRIENDS. No family or siblings. Friends.
What happens if you decide that you don't want to be their friends anymore? Are you completely stuck with them? Would you have to leave your horse?
Third, do you know how to tack up, groom, ride (I'm talking about walk, trot, canter, and possibly jumping)? Do you know what you would do if your horse got an illness or had to be put down? Do you know if you can afford feed, board, tack, grooming supplies, etc?
There are so many things you need to think through before even THINKING about getting a horse. Have you even thought about any of this?
This sounds like y'all were just talking one day and just randomly decided "Hey, let's get some horses!" I don't think this is a good idea at all.
Are your parents even okay with this?
This just sounds like a terrible idea.
Please THINK before doing any of this. Buying a horse without the right knowledge and effort can result in SEVERE injuries and is bad for you and the horse.