Of the common features that I was missing in my Pentax lens collection, two big ones come to mind. Weather sealing is one, but without a sealed camera the point is rather lost. The other is silent focus. That's worth having for creatures or circumstances where loud noises like a screw-drive focusing system are out of place.
Pentax has several very nice lenses with quiet motors, but generally not at the price point I had in mind (i.e. Cheap). I found a Sigma HSM 50-200 that would do nicely, but starting at 50mm was not what I'd had in mind when my 55-300mm is so good by every other measure but silence. Today I found the answer, and it's an answer that I have tried before: Amazon's used warehouse had a Sigma 18-50 f/2.8-4.5 in Good condition for $96. Good is good enough for me - so I bought it.
Another curiosity is that it competes* with Sigma's own 17-50 and 17-70mm models for shelf space. Both of those are well regarded but this lens' unique design is unmatched by them. And the price is no contest at all! *The competition is over in fact: the 18-50 has been discontinued while the other two live on.
As I said I've owned this lens before. I liked that its zoom ring goes the same way as Pentax, and opposite most other Sigma zooms that I have tried (including those 17-50 and 17-70 models). It's decent at closeups though it does not compete with a true macro. The lens is somewhat bulky compared to the DA18-55 but that's because it's f/2.8-4.5 instead of 3.5-5.6 - and the size never changes when zooming or focusing so its bulk never changes shape!
On days when carrying less is handy, the 18-50 + 55-300 covers my typical shooting range really well. Toss in the DA15 and coverage is even better! Add the DFA50 when closeups are needed. As long as you've done that, why not bring the K24? With that you can now leave the 18-50 at home!
Hmm wait a moment.. oh yes, I see it now: there's the confusion! :~)