As a novice rider, I bought a 7 month old foal.
9 years later, I still own "the foal".
Now that I'm an intermediate rider, have a diploma in Equine Studies and run my own boarding stable, I can say that it was the single biggest mistake I have made in the horse business.
I will never, ever, ever own a baby again. A green 3 yr old? sure. An unbacked, unhaltered yearling?, no way.
I was lucky. At the time, my sister took the foal and worked with him every day - just little 10 minute sessions - haltering, picking up feet, general grooming....getting him used to things.
And then when he was put under saddle, my best friend is a pro trainer - looking back, both the horse and I would have been in serious trouble if she wasn't helping us - she worked with the horse four times a week from the ages of 3-5. Add that cost up!
I do not mean to be a Debbie Downer - but this does not sound like a situation that will end well for you or the horse. The hours and hours it takes to properly raise a horse that is trained is simply not worth it unless that is your business.
And, I had a halter broke 7 month old foal. You are looking at a 1000 pound 1 yr old that has never had one on? Has it ever been handled? How is it with the farrier? Does it pick up it's feet? The fact that the person you are getting it from did not bother to halter train it signals to me that they haven't trained it for any of the basics - and now the animal is big enough to get you hurt badly.