Auto focus is not of much importance - I never use auto focus when shooting macro.
The older Pentax-M and K F4 and Pentax-A F4 macro lenses (check the
lens database) are excellent and available second hand from time to time.
Keep an eye on the market place - there is at least one Pentax-M 100mm Macro
offered for sale right now. Note that "M" lenses can only be used in manual exposure mode whereas the Pentax-A version can work with all exposure modes.
You are doing right by looking for a 100mm lens rather than a shorter focal length. 100mm (or konger) gives you more distance to the object you're photographing and hence easier to work with.