Hi Bill...
Pentax continuous AF - as you probably already know - lags behind the major competition. No Pentax model is ideal for subjects moving erratically back and forth. As a result, you need to bring quite a degree of skill to the party, and not expect the camera to do all the work (remember, folks successfully shot sports with manual focus lenses on film cameras back in the day
). That could mean pre-focusing ahead of where you think the subject is heading, setting an aperture to give sufficient depth of field for minor AF or MF focusing errors, waiting for the subject to move sideways rather than toward or away from you, and a bunch of other techniques and tricks. Nonetheless, plenty of people get *great* results for both birding and sports photography with Pentax cameras - but skill
is involved.
AF limitations aside, I guess you'd be looking for something with a fairly high continuous shooting rate and decent sized buffer. On that basis, the outgoing K-3II model (new or used) seems like your best bet - or a nice used K-3. I'm sure others will chime in with their thoughts, but that's what my gut instinct tells me
EDIT: I see Adam beat me to it, but with the same advice