First of all, you don't need to buy a digital lens for that camera. You can pretty much buy any lens Pentax ever made and it will work. If you're looking for a super-telephoto, you can buy anything that's made to fit the Pentax SLRs - digital or otherwise. Personally, I'd stick with the Pentax lenses, although Sigma, Tamron, and Tokina all make good stuff as long as you guy their high-end glass. And I think you're on the right track considering a 300mm or 400mm prime. At those lengths, most zooms aren't so great. The prime lenses will cost a fortune, but they will be beautiful. One exception, and a lens I've heard nothing but good things about, is the Sigma 120-300mm f/2.8. I have been coveting that lens for a couple of years. Here's a link to the reviews:

Sigma 120-300mm f/2.8 Lens Reviews >>

Note: the review page is for the Canon EF mount. But aside from the HSM focusing mechanism, the lens is the same. The focus probably won't be as fast for the Pentax. But it should be as good or better than the lenses you have now.