I would suggest a 40mm limited(not super fast, but great all around) or a FA 43mm limited (often praised as one of the best Pentax lenses ever).
Or! DA 35mm, DA 40mm XS, DA 50mm, Samyang 14mm, Samyang 85mm. You can buy two of these for under 500. The 40mm XS is very sharp, has low distortion, and can be used for landscapes. The DA 50mm is f1.8, which makes it great for low light situations and at 50mm can be used for pretty much anything. The DA 35mm is a little wider and can hold its own. The Samyangs are MF only, but are reportedly very sharp and their focal length sort of fills in gaps in the Pentax line.
Whats also great about Pentax is that you can mount older lenses, which can have great IQ but be much cheaper. The lens review section can be helpful:
https://www.pentaxforums.com/lensreviews/