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Actually, with your budget, you might be able to pick up a used DA* 50-135mm f/2.8 telephoto lens. It would be such a step up from a consumer lens like the 55-300 models (which are very good, but would be no match for the 50-135). Then you could add a 55-300mm later if you feel you need the extra reach (older models such as the DAL 55-300mm sell for around 100 dollars nowadays, and would be a great step up from the DA 50-200mm). It just depends on what you want to photograph, but for portraits and short to medium tele needs, the 50-135 would be great. I know quite a few people here in the forums who swear by it. Another concern is the SDM motor, which has been known to fail, but as far as I know it does not affect this lens as much as some of the others like the DA*16-50mm. And if you get a more recent model, that issue should be solved - and if it does happen, it can be converted to screw drive.
Here is a flickr search on it, sorted by what flickr thinks are the "most interesting" photos:
Search: pentax 50-135mm | Flickr