I have used the 18-55+55-200 bundle (early series, non WR, but same built and optical formula) for nearly 10 years. And I bought the 18-135 WR last year bundled with my K30 (same camera as K50, only difference is body styling).
My 2cts advices:
1/ K50 is the best APS-C DSLR you can buy at this price tag: excellent sensor, with high dynamic range and state of the art high ISO performance; outstanding ergonomy, with advanced functions only available on much more expensive contenders; more pixel than you need in most scenes unless you crop a lot; very good built quality, lots of in-camera JPEG tuning possibilities and very malleable raw files.
2/ the 18-135 on K30/K50 is a very efficient do-everything lens for all kind of photography: very sharp in the center, very nice colours ad contrast, well corrected CA, compact and buil like a tank, very fast silent AF. And for snapshots, thanks to its range, you always get the right lens aready mounted, ready to shoot. Only con is the corners are soft wide open and remain soft even stopped down above 70mm; thus it is not the best lens for shooting very high definition landscape or architecture details at focal lengths above 55mm (but better zoom lenses are more expensive and usually bigger). IMO, in most lanscapes, action shots or general telephoto, the 18-135 blurred edges are not an issue, they are hidden by the nice out of focus bokeh. On the other hand, Pentax choice of a very sharp center gives very pleasant punchy close-ups, with a kind of 3D subject isolation like with limited primes..
3/ the 18-55 is not better than the 18-135 between 18 and 55: stopped down, both are good and the results are very close; wide open, the 18-55 is more even, but the 18-135 is sharper in the middle. And 55mm is a little too short for many snapshots.
4/ the 50-200 is sharp from 50 to 100, has better edge sharpness than the 18-135 at focal length between 70 and 135, but is not as sharp in the center, and less contrasty. General IQ is good for the price tag, but the only moderate center sharpness needs to be helped by more accentuation in PP or JPEG tuning.
So, IMO, the 18-135 is the best choice to suit your needs. Later, you may complement your bag for a moderate budget by the DA50mm f1.8 for portrait and DA35mm f2.4 low light general scenes. Both are cheap and very sharp from corner to corner, producing high definition very contrasty pictures, and very resistant to flare.
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