That 12-24 is a fine lens; it's got great sharpness, but I hear the purple fringing is a bit nasty. But that 18-250 is intended to be a jack-of-all-trades kind of lens. That's the kind of lens that a photography newbie puts on his first DSLR and never takes off. Because of that incredible focal range (18 to 250), the image quality is a bit reduced overall; there's some severe barrel distortion and vignetting at the wide end, blurry images at full telephoto and chromatic aberration problems at all focal lengths.
You say you're eying a Kaf2 mount camera. That could be anything. Is it a K100? A K20? I'm curious because if you're getting the K20, then you likely have some cash to spend and my lens recommendations will change.
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If you want to cover your ranges for cheap then I'd recommend this(Staying away from any obscure lenses or manufacturers):
Sigma 10-20 - significantly cheaper than the 12-24 and it goes a bit wider. Downsides are that the image quality is very slightly lower than the Pentax and it's a 3.5-5.6 instead of a solid f4, but I don't think that will bother you. I know Pentax makes a 10-17 lens, but it's fisheye and I don't care for that type of look. This Sigma has very, very reduced distortions for a lens that goes down to 10mm.
Pentax 55-300 - Considering the low price and the large range it covers, this is a very good lens. Respectably sharp and the distortions are well controlled.
The kit Pentax 18-55 II - Just to cover the gap. Not to mention this lens will come with any K200 or K20. The II version is a real improvement over the original 18-55, and that's saying something because the original was fine. The II has increased sharpness (For use with the K20's 14MP sensor) and also, the Fringing, the vignetting and distortion have all been reduced. All that for only $50 more than the original lens. It's by far the nicest kit lens from any manufacturer.
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If you have some cash to spend, then my recommendations change:
Sigma 10-20 - Ok, so this stays the same. It's a really great lens that just happens to be cheap too.
Pentax 31mm f1.8 Limited - A dream prime lens. Blasphemously sharp and great characteristics all around.
Pentax 50mm 1.4 - It's a bit soft at 1.4, but there isn't a better 50mm solution for Pentax unless you dig into the past(*cough* Like I did).
Pentax 50-135 f2.8 - It's f2.8, it's weather sealed, it has great characteristics and it's reasonably cheap. I list this one as well as the 50 prime because the prime goes to f1.4 and it will look better than the 50-135 at 50mm, simply because that's its specialty.
Pentax 200mm f2.8 Prime OR the 300mm f4.5 - They'll take a better photo than any 70-300 or 55-300 at comparable focal lengths. They're both pretty good.
Note: I've excluded the Pentax 16-50 f2.8 lens because it's got some image quality problems which have left me unimpressed. If they release a version II, I'll reevaluate.