As a fellow 'bad-back' sufferer - I'm afraid I can't help! Sorry! All backs are different. I'm currently keeping a car going because the really important thing about it is the comfort of the seat. Depends, really, where the back pain is. I can only check by going and sitting in cars, perhaps you can try that with chair suppliers?
I'd recommend the sort of 'chair' that you actually kneel on. It gives you good posture, encourages leaping to your feet to pace the office, builds up the essential tough knee calluses, and generally does you good. If you try it and take to it (as I did) you will never regret it. If you try it and hate it, then that's a pity, because it would have been good for your back. Best wishes.
Atheist, I read recently that those ergonomic chairs which were all the rage a while ago are not great for back problems.
The article said any chair is ok as long as you stand up and walk around regularly.
Cloverjoe; I tried one gingerly many years ago and was immediately hooked. In my experience those seats are far better for you than the typical office slump-chairs. But it's like vegetarianism; some people simply hate it at first sight and won't be changed. That's what humans are like. I have no ambition to change people's views, I just make suggestions in the hope that they might bear fruit.