Thanks for all the useful tips! The 28-105mm indeed looks like the only acceptable quality lens.
I've put the PentaxForums ratings to a table:
lime green - lenses I currently use
light yellow - considered readily available lenses
I wouldn't personally rate SMC DA L 18-50mm F4-5.6 DC WR RE autofocus 9.4 (it is more like 6 or 7 in my case) or SMC DA L 50-200mm F4-5.6 ED WR sharpness 9.0.
On the other hand SMC 24mm F2.8 is probably underrated.
Another highly rated lens which is readily available for me is the SMC A 35-105mm F3.5. Unfortunately 28mm or 35mm at the wide end make a difference and it's manual focus only.