If back probs are a concern, I would think you would seriously want to look at full suspension. Absorb some of that extra hit on the rough stuff. A good dually in the sub 1k range will be tough though. Would have to shop used.
This X2 as well. If you do get a hardtail get a steel frame. I ride an aluminum hardtail that is awesome but it sends every bump into your ass versus a steel that absorbs some of the impact. They also make a suspension seatpost if you end up with a hardtail..