Check out the rimfire section over at the Ruger Forum. All you'd ever want to know about Ruger .22 autos can be determined by running a few searches.
I have a MK-1, and have bought a couple of after market mags made for the MK-2 that work fine. There are some frame variations between the older MK-1's (1950's & 60's) and the newer MK-1's (1970's). I've heard the mag issue is the placement of the follower pin on the side of the mag. But it can be removed and re-installed on the other side with little difficulty.
The big difference between the MK-1's & 2's is the slide stop feature on the MK-2. But there is a way to modify the magazine so the bolt will stay back after the last shot (search the Ruger Forum for that too). Most everything else is cosmetic, add on's and barrel weights/lengths. The MK-2 style is also available in a polymer frame version that approximates the grip angle and safety/slide stop controls of the Colt 1911 design (if you want a cheap ammo understudy for a 1911).
To your final question - no, there is nothing wrong with the MK-1's. About the only way to wear one out is to leave it soak in salt water for months or use it for a hammer. I've had one since 1976 and have run many thousands of rounds through it. The finish is a bit worn, but it shoots as good as it did new. The MK-2's are just available in a wider variety of finishes, sights, barrels, etc. if you are looking for particular extras.
; they're still lots of fun, though.
