I had an epi. I had gone in to the labour ward determined not to get one because I was frightened of the thought of something in my spine and I wanted an unmedicated birth, but at 7cm dilated, I caved. And it was fab. I had minimal pain until it was time to push and even managed a nap to rest up a bit so I had bounds of energy for the last 90 minutes of pushing (when the epi was turned off and i felt every little thing).
I had back pain afterwards for a few months, but every woman who has had a baby has back pain as her spine goes back in to place so I dont think it was epidural related? Anyway, it was not bad enough for me to consult a doctor about it and I felt completely normal again after a couple of months.
I would have an epi again. In fact, I am frightened at the prospect of labour without one!
In your case, I would definitely ask the doctor about the pros and cons because of your unique back issues. I dont think anyone, even those who have had an epi with similar back problems would be able to advise you properly because it all depends on your medical history.