In the Pentax line, there are different versions of the consumer lenses. In the 50-200 there is the regular lens and then the WR version. If your goal is to match the weather resistance of the camera then make sure you are getting the WR version. The 55-300 is not WR. Optically, the 55-300 is supposed to be slightly better than the 50-200. However, I own the 50-200 and when I want to go light (normally I would use my 70-200), I love it. I don't know if I just have a good copy, but I have no issues with it.
Now the 18-55 has three versions, DA-L, DA-AL and the WR. The DA-L has a plastic mount, no hood and no quick shift. It is a cheaper lens, construction wise. The DA-AL is the full version with metal mount, hood and quick shift. The WR is the weather resistant version of the DA-AL. Optically, they are identical.
I hope that helps to explain the lenses. Just make sure you are getting what you want. As I stated earlier, the main difference between the 50-200 and the 55-300, is that the 55-300 is slightly better optically, not WR, longer focal length and slightly heavier.