The K 55mm is a bit of an odd one.
There are f1.8 and f2 versions which are in fact identical - the f2 is rigged so that the aperture can't open as much.
The f2 was only sold on the Japan-made K1000, the f1.8 was a kit lens option on the KM/KX/K2. The 50/1.4 was the mid range with the 50/1.2 at the top.
The other option in camera terms would be to look at a P30 or SFX/SFXn. They rely on batteries but providing you find a good one they're reliable, and are now ridiculously cheap. The SFX/n can run from AAs with the replacement grip. Both can use any K lens which covers full frame from 1975 to the present day, although they're best with FA or earlier lenses which have an aperture ring.