Originally posted by jimH Being relatively new with photography, as you say you are, I'd suggest that you do the most that you can with the kit 18-55 and 50-200 mm lenses that came with your camera. Reason being, the on board flash or any flash dedicated to the Pentax system will work well with the automatic lenses. I have a couple of fast f 2.8 lenses in the ranges of my kit lenses, but for various reasons, I'll go back to my kit lenses at times, particularly if I'm concerned with weight..
I agree with
jimH, there is absolutely nothing wrong with your current lenses and they should be able to produce very nice images...
I think you should use your 18-55 for an entire day taking images near the 50+mm end and don't move it from there, see how you like it, see how much you need to walk around your subject for composition, etc etc etc... it'll give you an idea what its like with a 50mm Prime lens.
FWIW,
radium22 mentioned a 35mm, I think this is actually a great focal length for everyday use. Over time, I've found I use a 28 or 35mm more than my 50mm lenses.
Having a nice fast manual Prime lens does not guaranty great photos but it is fun trying
Good luck...