Plumfoodie
Paleo maintenance rocks!
I agree with Donzy, start with the small bell that came with the set and see how you get on to start with. BUT if you are still having active back pain I would start with some gentler core exercises first for a week or two, and stretches, to see if you can resolve that first. If you still have an injury you don't want to aggravate it. If going to a chiropractor is an option for you I'd suggest that - mine is always very helpful and gives me stretches and exercises to target the trouble area. Google might have reviews for some chiropractors in your area if you don't know anybody who can recommend one.
When you do start kettleworx, I'd suggest trying each exercise without the bell first so you can see whether anything is going to twinge your back and then avoid or modify those exercises when you come to them in the routine. And stretch really well afterwards - the short stretch at the end of the routine is ok if you're not having any issues.
The 'preliminary' spinal stability/core exercises suggested in my kettlebells for women book (ie to do for a week or two before you actually start a routine) are:
Bridge position - Bridge Progression 1 - YouTube
Quadruped progression - Quadruped Progression - YouTube
Neutral plank position - Best Plank Exercise Routine Ever - YouTube (lots of good info about how to do planks, and many variations - start with the 'modified plank' at the beginning)
Side plank position - Side Plank Progression - YouTube (these are present in the previous video too but this one shows an easier variety at the beginning and then do the harder ones as you are able to)
(just a thought - are you drinking enough water? I get kidney pain if I'm dehydrated - it is usually only on one side of my back and I guess it's at what I would call the boundary between my middle and lower back)
