If I've had damage (nerve) to my lower back is it a good idea to have an epideral for a c-section?

Yes. If the epidural is performed properly it should not further injur the nerve.
Discuss it. The extent, the kind, and the location of the nerve damage should influence your decision. Discuss all of the aspects of injury and disability with the anesthesiologist the nerve damage has caused. If you feel that s/he can not appreciate your concerns, consult a neurologist for recommendations. An epidural may be preferable to general anesthesia when all risks and outcomes are considered.