I have three Michael by MK bags & love them all. None of mine have a zip closure though, they're all magnetic snaps. I think they are a great buy--all of mine are holding up extremely well, and I adore the hardware and the leather.
To help the zipper slide, I've heard you can rub beeswax on it. I've never tried it myself, so I'd try it with caution, but it might be worth a try. As to whether or not it's too heavy for you, only you can decide that for yourself. I also have back issues & I try to be really cautious of two things:
1) Buying a bag that's very heavy when empty
2) Carrying too much stuff in my bag
If you think that it's what you're putting in the bag that might make it too heavy, look to see where you can minimize--makeup bag, wallet, etc. Make a habit to clean it out each night so that you aren't carrying around things you don't need. However, if you think it's the weight of the actual bag itself, I'd say you might want to part with it, or at least not carry it on a regular basis. HTH!