There have been several recent threads on the topic including
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/99780-next-lens-nifty-fifty.html. 50mm is regarded as the optimum focal length for portrait. Some might prefer 40mm, other 55 mm ...
A number of Pentaxian experiences have highlighted 4 prime lenses that can be recommended:
- the Pentax 50mm f1.7,
- the Pentax FA50mm f1.4,
- the Voigtlander Nokton 58mm f1.4, and
- the Carl Zeiss 50mm f1.4.
All 4 lenses are available in Pentax mount. The Pentax 50mm f1.7 is an older MF lens, highly regarded' it can be purchased second hand at very reasonable prices; it is worth to consider for your small budget. The Pentax FA50mm f1.4 is a nice AF lens, reasonably priced and well regarded.. The Voigtlander Nokton 58mm f1.4
is a MF lens with a super image quality and superb built quality; very well priced (same price as FA50mm), very solid and ideal for outdoor shooiting. The Carl Zeiss 50mm f1.4 is a MF lens, that is the Rolls-Royce of the 50mm: expensive but the reference.
Based upon your budget, the 50mm f1.7 is possibly the best compromise/optimum.
Hope that the comments will help...