Yeah, 70-200mm is a good bet. The HD DA 55-300mm is not as high quality, but it is still acceptable for a lot of people; especially due to its much more accessible price point. DA* 50-135mm is popular, as well, but might not be long enough if you are far away.
DFA 150-450 is interesting, as well.
The absolute cheapest is a used DA L 55-300mm, which is optically almost the same as the HD DA version. A little cheaper is the DA 50-200, but this one is a hair worse than the 55-300mm, and it doesn't have as much reach. Though, for some users its enough. I think the 55-300mm is better than the Tamron and Sigma 70-300mm lenses (
here and
here), though I'm sure you can find comparative reviews on the forum. There was a thread recently about these, with lots of sample photos and comments
Here are links to reviews:
https://www.pentaxforums.com/lensreviews/hd-pentax-d-fa-70-200mm-f28-ed-dc-aw.html https://www.pentaxforums.com/reviews/hd-pentax-d-fa-150-450mm-prelim/introduction.html?src=lrdb https://www.pentaxforums.com/lensreviews/SMC-Pentax-DA-Zoom-Lenses-c43.html
Some of these lenses have "official" PF reviews, as well as general user reviews. And you can check out the Sample photo gallery to see some user photos.
It comes down to your budget. With telephoto, there are big differences in price; and better is more expensive. Better means the photos are sharper, have more detail, AF is better, and low light performance is better.