The K-50 is a great camera with a great quality reputation. The loose noise Old4570 talks about is probably just the shake reduction mechanism, not a problem at all. I have a K-30 and love it! A used K-30 is a great idea, too.
If you can afford it, get the kit with the 18-135. A nicer lens than the 18-55, and more useful. If you can't, by all means, get the 18-55 kit. It's not much more expensive, and it is worth every penny for the kit lens.
You will need to change some settings to use your old manual focus glass on the K-50. There's an article about what to do
here
Since you only have one M42 lens, if you don't already have an M42 to K adapter, I recommend the cheap fotodiox one. However, with this cheap adapter, you must remove the locking clip, because it is junk! Then put it on your lens kind of tight, and then just put the lens on and off like a K-mount. The lens won't lock in place, but I've never found it to be much of a problem. Just don't focus all the way out and keep turning, you can remove the lens accidentally. If you are even a little bit careful, it's never a practical problem, though. Here's my favorite shot with my Tak55 (shot with my Kx)