Affordable Pentax primes, full auto: DA 35mm f2.4, DA 40mm XS, DA 50mm f1.8
Affordable Pentax legacy primes: M 50mm f1.7 and M 28mm f2.8 (or A series, which has auto aperture but costs more)
New K-mount, but full manual:
Horus Bennu 50mm f1.7,
Zenitar 50mm f2.0
Sigma, brand new: 30mm f1.4 (various versions, older ones are cheaper, but not quite as good as the latest art version)
Samyang also makes a 35mm f1.4, but its not super cheap and it has no AF.
Any of the ones mentioned will give you significantly better image quality and low light performance than the kit lenses. I recommend you get one of the Pentax DA lenses, unless you enjoy taking you time and doing manual photos