Hello Schnur07, Welcome to the Forum!
The first lens that came to mind after reading your post is the DA 35mm f/2.4 AL.
For about $200 USD new, (about $150-170 used) you will have a tack-sharp, fast A/F normal-view prime that will serve you well as a walk-around, street, candid and landscape lens. It is very small, light and a bargain to boot!
Here's the PF lens reviews;
SMC Pentax-DA 35mm F2.4 AL Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database
And for more info and lots of sample photos, check out the "Plastic Fantastic" thread.
I believe it will work as well on your K100D as well as any modern lens, for even faster A/F, you would have to upgrade the camera body.
FWIW, the FASTEST and most accurate A/F I've ever seen on a Pentax lens isn't a prime, but the F 35-70 f/3.5-4.5 zoom. While it lacks the great low-light capability of a prime, in all other respects it is a fine all-purpose zoom with a "Macro" (close-focusing, actually) function at the 70mm end. About $50.00 USD on eBay or other online sellers.
Both these lenses use a 49mm filter/hood size, a near-standard Pentax size. Saves quite a bit of money and space in accessories like CPL's and hoods!
JMO,
Ron
Last edited by rbefly; 01-03-2013 at 09:45 AM.