We just bought a horse for my daughter after looking for a long time as well. The way her trainer explained it is that you buy the horse she needs now and when she moves up, if the horse can't move up with her, then you sell him and buy another one. The trainer said we can't expect to buy one horse to carry her through the stages of her hunter/jumper "career". I was shocked at first. I always pictured buying one horse and her having that horse for years. Apparently it doesn't work that way.
When you're green and learning, you buy a horse that has been there, done that. By the time you get a little experience under your belt, that horse will be ready to teach the next novice and you will be ready for a more advanced horse.
A 2' horse and a 3'9" horse are not the same horse.
Sorry... hope I didn't burst your bubble. But this horse could be exactly what you need right now. You just might have to trade him in for a different model later. :(