500 or 600, I'd say the Sigma range would cover it for you, but even 2nd hand, its going to be quite a lot of money. Also remember the hassles in terms of weight and setup/portability with those bigma's., and you will DEFINITELY need to tripod it up very sturdy.
If you are willing to consider your 1.5x crop factor, then a 300mm would "seemingly" produce 480 mm...
I have a Sigma 70-300 APO, and I am really happy with it, I don't do surfers really, but I do airshows and offroad bike races, and it does wonders.
But shop around indeed, anything new for a pentax mount will work on your camera