+1 to both Sandy Hancock's and Na Horuk's suggestions. If you don't mind the manual focus (and I'm guessing you don't, since you have the A 50mm f/2) then the A 28mm f/2 would fit your described needs/specifications. I think a lot of people forget about the DA 40mm XS and the fact that it doesn't have to live on a K-01, plus it's surprisingly affordable. The only downside, at least if the PF reviews are any indication, is the XS 40mm has a downright irritating lens cap. If it were me making this decision, I would not get the 40mm only because it's so close to the 50mm prime focal length you already have.
And while we're on the subject of 40mm lenses, don't forget, there is also the SMC Pentax-M 40mm F2.8 which is a nice pancakey lens and can be had for a nice price, and the Vivitar 40mm f/2.5 VMC Pancake. It looks like the M 40mm 2.8 get's OK ratings, though not as stellar, obviously, as the 40mm ltd. - Interestingly, on
Bojidar Dimitrov's Pentax K-Mount Page the M40 and 40mm ltd. have nearly identical optical formulas, but it looks like the newer limited lens has been tweaked slightly. As for the Vivitar, I don't know how common that lens is; the only reason I know about it is because it once showed up in the Marketplace.
I have a DA L 35mm f2.4 and it is a nice little lens if you don't mind the plastic mount. Digging around Bojidar's page showed it appears to be a tweaked version of the FA 35mm f/2. I don't know if f/2.4 is fast enough for you, but the focal length is a comfortable distance from your 50mm prime to make it worth swapping lenses in the field. I find the image quality to be pretty good, certainly decent wide open. I bought mine at or near the lens' original price, about $240, when it first came out, but now you can find them new for less than $200.