For an all-around zoom AF lens, the best options are:
* Tamron 18-250mm / Pentax DA18-250mm, or
* Sigma 18-250mm OS HSM.
The former is discontinued by you can find some excellent second hand copies. Both keh.com and adorma.com had some used copy about US$360 (last week). The DA18-250mm is a rebadged Tamron 18-250mm, and both are regarded as among the best all-around zoom lenses across all brands. The lens is very sturdy and well-regarded as an all-around lens by most Pentaxians.
The Sigma is a newer lens, slightly more expensive. It has in-lens OS which is redundant with the in-camera IS of the Pentax bodies.
A newer alternative is the Petnax 18-135mm WR but the reach is smaller and the price for the WR is consequent.
For completenes, there are some older generation 18-200mm lenses around but the IQ is below the above lenses.
Hope that the comments will help....