Well right again as i work in a sort of photo type shop and sell people printers and cameras on a daily basis.
I don't have any experience of the model itself, but i do know we as a store stopped selling the smaller pentax range of cameras because the quality of picture the camera took was poor and build quality questionable.
Again i'm afraid not sure about the lens mount but if the 500mm lens you are talking about came off a film SLR camera then the focal length would change to 650mm. Using non dedicated lenses designed for film cameras on digital bodies increases the focal length by a factor of 1.5.
Unless you are set on the Pentax i would have a look around for a second hand Canon EOS 300 or similar. The build and picture quality is excellent plus the massive range of Canon accessories. The digital market has followed in the footsteps of the film market with Nikon and Canon producing the best equipment by far.
I sold my Canon EOS1 film camera last year after many years of faithful service and bought a Nikon coolpix 8800 digital. Not an SLR as i got fed up of curvature of the spine when i carried my olf film equipment everywhere . I can recommend either of those brands.
Hope that helps
"They couldn't hit an elephant at this distance."
Famous last words of General John Sedgwick
May 8th, 1864