For dental, walkabout and family, the K-X should be fine. How rough is the hiking that you're likely to do with it though? If you're talking rain, sleet, snow and desert, the K-7s weather resistant metal body might be your better friend - for example I knocked my tripod over hiking last year with the k-7 and I should imagine that a plastic camera might not have weathered this challenge.
In terms of classic old lenses, look for Pentax SMC-A or Pentax SMC-M. Both of these are manual focus, the -a is somewhat more convenient. You can also look at older, Takumar, which have something called an M42 mount, for which you need to buy a converter. Pentaxforums has
lens review section you'll probably find useful.