Originally posted by pathdoc OP has a K-5, Sandy; the PLM lens won't work on those.
PLM lens work on every Pentax camera with at least KAF3 mount.
Only stopping down isn't possible. So shooting with open aperture is possible. Since the aperture is slow, open aperture would be recommended in some settings without direct sunlight.
Buying a used Sigma 50-150mm EX DG HSM would be the best option considering focal length, speed, aperture and price.
Other options are waiting for the new Pentax D FA* 85mm 1.4, which maybe will released until photokina 2018.