There have been other earlier threads on the topic and you received already solid suggestions.
Many see the 50mm focal length with large aperture (small f) atypical of a portrait lens. The large aperture allows you to work indoor and outdoor including in lw light conditions. In that categrory, there are a range of interesting length including:
- the older Pentax 50mm f1.7,
- the FA 50mm f1.4 {
Pentax SMC-FA 50mm f/1.4 - Review / Lab Test Report},
- the Voigtlander Nokton 50mm f1.4 {
Voigtlander Nokton 58mm f/1.4 SL II - Review / Test Report}
- the Carl Zeiss 50mm f1.4 {
Zeiss Planar ZF T* 50mm f/1.4 - Review / Lab Test Report}.
The list is not exhaustive. The above lenses are some well-respected lenses in their own category and price range, and they are available in Pentax K-mount. I added the URL of the independent review by Photozpone.de beside.
The Carl Zeiss is the basic reference: i.e., a 'Rolls Royce', but expensive. The Nokton 58mm is very well respected, excellent image quality, and within your price range. For the same price as the Nokton, the FA50mm has AF, but the image quality is s par below. The older Pentax lenses are still good values, but can only be found 2nd hand.
Since you already have 2 kit lenses, both AF, I would encourage your to expand your photographic techniques by selecting a MF fast prime lens, like the Voigtlander Nokton 50mm f1.4 (or the Carl Zeiss if you can afford).
I add that I bought the Nokton for outdoor low light shooting. The lens si very rugged and well-suited ofor outdoor. But I love the lens and it found to use it much more than initially thought.
Hope that the comment help...