I agree with everyone else, but in saying that I went out to try a horse one day and it went perfect for the owner, even pulling himself into a proper working frame without the owner commanding him for it, then I had a friend of mine get on him [I have a bad back too even though I'm only 15, scoliosis :S ] anyway, she got on him and she can ride the most mongrel of things, she can hang on like hell! and he went smoothly for her to so I thought well heck I'll get on him, but before I had finished thinking that thought, the owner tightened the girth on him, the girl rode him around a bit, and you could have said a rocket had exploded in his bum.. he went PHSYCO!!! and buck after buck after buck, they got bigger and bigger and he finally threw her off, onto her neck on the hard ground and hurt her back, landing RIGHT beside the fence :S, she would have clipped it if she got any closer whatsoever, she hit the ground that hard the peak ripped off her helmet and threw it a few metres away.
So, long story short, I think because of your back, like mine, next time you go out to buy a horse get a friend to get on it after the owner has to test it out a bit before you get on, just incase he/she does what the horse did that I went to see, because if it was me that had landed the way she did, I would be in ICU right now probably on life support :S.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms