Go for the primes.
A prime lens gives substantially better IQ than a zoom lens for comparable price, because the zoom lens needs to designed with some compromise over the focal length range.
The 18-135WR is overpriced in my opinion. Since you have a K-x, you do not need WR and you could do as well with a DA18-250mm. (An used 18-250mm is about $360 with a much greater focal length range.)
Among primes, there are a huge choice for Pentax mount, starting with Pentax primes. Start from the beginning: get one prime around 30-35mm. This focal length (30-35mm) is very useful and convenient. Later you could consider another primes around 50-70mm.
I have both a 18-250mm and a series of primes. I love both: the DA18-250mm for its flexibility, and the primes for their IQ. Since you want better IQ, and have already a zoom lens (18-55 mm), I would highly recommend to go for some primes.
Food for thoughts.