Check around on the online auction sites. For Pentax the best macro would probably be the legendary 200mm (very expensive). The best 100mm macros would probably be the Vivitar1, Kiron or Lester Dine 105mm 2.5 or 2.8 lenses (despite the different name, the optics is the same in these). These are among the sharpest macro lenses ever produced for any camera. The trick is in finding them at a reasonable price. As some people are not aware of their reputation, you might be able to get them at a good price sometimes. Last time I bid on one with the K-mount it went for about 220 euros. From Pentax 2.8 lenses the older FA is supposedly optically better (but significantly heavier) than the modern DA version.
It all comes down to how much macro photography you are planning to do. I think the f4 is a nice lens to start, but you might get the LBA soon for smth. faster. Also consider your shooting style. If you want the whole subject (an insect for instance) in focus you will use it at f11-f16 anyway. On the other hand, I use my macro lenses often wide open (f2.8-3.5) to get selective focus on a small flower for instance.