Here's hoping that a couple of more affordable and lightweight constant f4 full-frame lenses are on the way, maybe these will be the ones given the * designation?

300mm is a pretty long lens already on APS-C - using the common benchmark of 35mm equivalence that's 450mm. Anything longer becomes pretty tricky to shoot handheld even in good light. Pair a lens with good centre sharpness at 300 such as the 55-300 with a high-res DSLR such as the K-3 and there is also good scope for cropping if you want to go longer.
Even with my K-30 and 55-300 combo I have been really impressed with the amount of detail that was not visible to the naked eye that you can get out of centre crops of images taken at 300mm.

I have been considering this lens, as I don't have the funds for the 31mm Limited and with Sigma's cashback it is down to £225 at some places.

However, I am wondering why so few retailers offer it in Pentax mount? For example my go-to shop for Pentax, SRS, only stock this lens in Canon and Nikon fit.

I know that there are some reported issues with AF for this lens. Are they so bad that retailers don't feel it is worthwhile selling it I wonder?

Now showing as £279 which seems like a fantastic deal to me. I already have this lens and can attest to its image quality, and also its very pleasing "Limitedesque" build quality.

link

I didn't buy my copy there, but have bought from them once with no issues, visiting the shop in London in person.

I'm in a similar boat, I upgraded from a K100D to a K-30 earlier this year, and I now notice a much bigger difference in sharpness between the images produced by my DA 18-55 AL (mk 1) and, for example, my DA 40mm XS.

I like the versatility of this range, so am looking for something similar but with better IQ. Obviously more zoom range at either end would be welcome as would a faster aperture.

I have looked at the:

New Sigma 17-70 f2.8-4 C - £349 - looks a good option but a brand new model so no discounts available yet.

Sigma 17-50 f2.8 EX - £319 - have heard very good things and didn't this used to be more expensive? Less range though.

Tamron 17-50 f2.8 - £279 - also well reviewed but maybe not as well-built as the Sigma and now hardly any cheaper?

Pentax 16-45 f4 - approx £160 secondhand? - seems to be a well-liked lens, a touch wider which could be good. Only used copies available though, and some reports of barrel loosening with age impacting sharpness?

Pentax DA 18-55 WR - £60-ish - WR is a plus to go with the K30's WR, not sure how much of a sharpness upgrade on my Mk1 18-55 though?

Pentax DA 18-135 WR - £345 - WR, much bigger range, good reports of IQ. No faster though and expensive for a 'slow' lens.

Sigma 18-125 - £229 - again much bigger zoom range but no faster, would IQ be noticeably better than the 18-55?

Decisions, decisions...

Any thoughts or advice would be much appreciated.