I'll add my voices to those that say that autofocus is a bit silly for macro work. For the same money you could probably come close to getting a great manual focus macro AND a Sigma EM-140 DG TTL Macro Ringlight Flash for Pentax.
There is hardly such a thing as a bad macro lens. The longer the focal length, the farther away you stay from your subject (important if it is skitterish and flies off).
If you are worried about making the right decision or want to try several before settling, then I recommend buying used. You can always turn around and get you money spent BACK (or maybe make a profit, if you find a great deal).
I've been through a lot of them, but the one that will be buried with me is the (pretty darn rare) Tokina AT-X 90mm f2.5 with 1:1 matched extender. This is an "A" lens in Pentax mount. I call it the Bokina II. The original "Bokina" is the Vivitar Series 1 model of the same 90mm f2.5 with 1:1 extension, designed & manufactured by Tokina. They are not the same lens, but both are SUPERB, if not inexpensive.