IMO the 7 mag has a snappy kick and would be at the bottom of my list if I had back problems
A 243 running 107 or 115 dtacs would be my first choice to fit your needs
I ran all glock factory parts on the slide the biggest thing that bothered me is when the slide is on the frame it looks like you have a huge gap in between the two most of this is due tO the big bevel they put on the bottom of the slide
I put a LW slide on my G17 and it fit just fine but it had a few rough spots and a few burrs from machining nothing a tinkerer couldn't fix just remember you get what you pay for