Before you get a handle, consider a Dillon tall mount or the Inline mount. I built my benches tall, so I don't have them, but I think that with a back issue being able to stand up straight or sit on a bar stool is more useful. Get yourself an anti-fatigue mat for the standing if you don't have one.
Long verses short handle is harder to say. Long will reduce muscle effort, and I find that useful with priming, but it does reduce feel and if forces you to go through a greater range of motion, which may be an issue when sitting.
If you have a physical therapist for the back, they may be able to advise you on this. It might be they think the greater range of motion while standing would be good for you. My gut reaction is long handle, tall mount, anti-fatigue mat, but YMMV.