He sounds like a good choice, so long as your prepared for the responsibly of owning.
I wouldn't worry too much about the back issue, especially being a lesson horse. Probably a saddle fit issue, or maybe someone plopping a bit too hard on him. He could have even tweaked it playing a little to hard! So long as it's not a recurring issue I don't see a problem.
If you have to move away and couldn't take him with you he sounds like a versatile enough horse that you could lease out. Many people out there looking for solid packers for children or just an easy going trail mount! Worst case you could always retire him. I've looked into it and in my "area" (anything drive-able/day-trip-able) it ranges between $250-350 for full care.
As for his responsiveness, it's likely only a matter of training. It's amazing what some of these "dead head" lessons horses can do when a trainer get on them and pushes all the right buttons!! That should fade with time. My horse was a lesson horse, and a crappy one at that. She was hard to control, would constantly swap leads at the lope and would spin if you took your inside leg off or added to much outside leg (she is an ex reiner). Everyone hated riding her, even the other instructors. I got on her and she was a DREAM. Hence why I have her now!
One thing I would be concerned about, or conscious about, is his age. Older horses typically cost more money, whether it be for shoes or supplements. You may also be looking at him slowing down in several years. If that's not a problem for you, no worries. But if your the type that wants to show or do all day long trail rides over rough terrain this horse may not be a wise choice for the long run!
Lastly, vet check, vet check, vet check!! Don't leave any stone unturned. You'd much rather spend $1000 upfront on the check then LOADS more, plus the heart break, later on. Posted via Mobile Device