I probably would not buy an adapter that had a lens element to allow infinity focusing. I would worry the resolution would be degraded. I believe that was the concern with using the older Canon FD lenses on the EOS bodies. But I do have a Pentax kit and when I want to use the lenses on a digital body I have a full frame Canon body for that plus for my Leica R lenses.
As for Pentax lenses I would like to try on a Nikon body I would choose the original Super Takumar 50mm f1.4 lens with the radioactive element. That's a super lens and was one of the best in it's day.