My wife bought her first horse from the breeder/trainer down the road this way and it worked out very well. They offered a very nice agreement that my wife would pay $$s a week for a year that included paying for the horse and pasture board. The horse stayed in the same pasture/herd and the breeder continued the same feeding (hay+grain), vet, and trimming scheduling. Hay and feed were included in the pasture board, and my wife paid for the vet and trimming. They also included a one hour lesson every week and access to the barn, ring, and trails. My wife also had the option to lease out the horse, rent it out for trail rides, or sell it. The only conditions were that the horse had to stay on the property and the breeder kept the papers until the horse was paid for.
Before the year was up, however, my wife's birthday came around and she came home one day to a fully paid off horse waiting to see her