9MM carbines are a lot of fun. They are a blast to shoot. Light recoil, cheap to feed and generally pretty accurate too.
As to which to get, there are a lot of choices and price ranges. A couple of friends own the Hi-Points and they are happy with them. I've shot them and they are OK. If you are looking from a price perspective, they are tough to beat.
Next would be the KelTec SU2000s. They are light, compact and relatively inexpensive. They used to come in a variety of models for the various mags out there, Glock and others. I've shot a couple of them, but I don't own one.
The Ruger PC9 mentioned above are no longer made, but if you can find one at a decent price, they are great little carbines. I own 2 of them. The second was a police trade-in that I found for under $200 a few years ago. The Ruger P89 mags are easy to find and relatively cheap.
The Beretta CX4 Storm is around the price of an AR. The trigger is not the best, but for me the ergonomics are pretty much on. I like the feel of it, and while it's not an AR, it's NOT an AR. Meaning, the ergonomics are different, the feel is different, but that all depends on what you like or don't. It uses (or can use) the 92/M9 mags which are plentiful and inexpensive.
MP5 and clones, are a lot of fun. But they are pricey compared to the AR9s and other carbines. I do own one. Unfortunately it's one of the Special Weapons examples.
UZI's and clones, can be fun, but can get pricey. The folding stock is uncomfortable in most uses and I wouldn't recommend it for you.
AR9 platforms. They are getting cheaper and there a lot of options out there. It's an AR which is good if you've ever worked with one. The mechanics and manual of arms are simple. They are accurate and fairly light. I own a Bushmaster (old gen) Carbon-15 model which is very light as it's a polymer upper and lower. I own the Just Right in .40 and have the conversion kit for 9.
I've also owned the Cobray Mac 11, but it was the pistol version and I sold it.
I've fired the CZ Scorpion pistol, but not the rifle. It shot well, but I would like to fire the carbine version.
I'd suggest you handle a couple of different ones and see how they feel to you. If you know anyone with them, try them out, take a few shots with it.
Good luck with your continued health.