The 50/1.7 is regarded as better than the 50/2. I use the f/2 though, and it is good, just not anything special, IMO. The adapter is removed by using the the tip of a ballpoint pen or similar to push in the metal spring clip that's located on the left upper inside of the adapter (looking at it from the front). You can also remove the clip and leave the adapter on the lens and use it like a normal bayonet lens. Just be a little careful, as it won't be locked onto the body.
I think it's simpler to just buy a K-mount 35mm though, as they are inexpensive in ordinary focal lengths and apertures. Most are less than $100, while some are less than $50. I bought a 28/2.8 SMC Pentax-M for $35 from KEH. I got an 80-200/4.5 zoom off ebay for $50.
You could get say, the 135/3.5 for very little money, and with your 35 and 50 have a nice basic setup. Or you could go a little wider with a 28mm instead. If you find you want a longer lens the 200/4 is excellent and inexpensive. Third party lenses by reputable makers are even cheaper, and some are very good. Some of their lenses are very well respected.
One thing about it, you can get basic lenses so cheaply now that if you change your mind on one, you can sell it on and lose very little.
As stated above, if you only use the 50 to start out with, and really learn what that lens can do for you, you will better be able to use any other lens you acquire. I did that when starting out: I had only my normal lens for the first few years and really wrung it out before getting more. And if I could only have one lens to use all the time the normal lens would be it.