I personally would actually not bother with the older Pentax 17-25mm fisheye zoom. If you are OK with using a lens that doesn't give you the "full fish" effect, I would recommend the Zenitar 16mm fisheye, which you can get straight from Russia via eBay for well under $200. (When I bought mine, it was $145 or so, I think it's closer to $175 now.) You don't get the full 180' fisheye, but you do get a little bit, and it's a terrific little lens with very nice image quality, plus it's faster (F2.8). It's manual focus (not a big deal because the depth of field is huge) and manual aperture (not really a big deal - you shoot in Manual, change aperture with the ring on the lens, and press AE/L when you want to meter the light) but it's a lot of lens for the price. Very, very fun. I can post some photos taken with this lens if you want an idea of how it looks. The shots can also be "de-fished" if you want to use the lens as a very wide-angle one.
The next step is the Pentax 10-17mm, this will give you the full fisheye effect, as well as autofocus and autoaperture.
Those are, IMHO, really the two choices to consider. The older Pentax designed-for-film fisheye zoom doesn't make much sense for a DSLR compared to the Zenitar in my opinion. There is also a Peleng 8mm circular fisheye, which on a film camera gives you a complete circle with black all around, and on a DSLR, gives you a unique fisheye look with black corners. This is a little more expensive than the Zenitar. Sigma announced a 10mm fisheye for digitals late last year but I don't think it's available in a Pentax mount. Although that may change, they (and others) are releasing more lenses with Pentax mounts now.