Well, my mind is pretty much made up now.
DxO have now added the K5 to their database. The sensor scores 82, which is the highest APS-C score so far - the next highest ranking APS-C is the Sony 55 with 73. The only higher ranking FF mainstream camera is the Nikon D3X with a score of 83. See
HERE
The dynamic range is truly staggering 14.1 EV compared to 12.2EV for the D300S, and 11.7EV for the 7D for example. You have to go to FF for better low light performance.
I see that there is £120 cash back on the weather resistant twin lens kit too, making the body and two lenses less than £1100. Which is good for something you can use in all weathers without a worry. Or a plastic bag.
As for a 50mm - well there are FA f1.4s for sale on eBay regularly, and I think Amazon still have some new ones. I bought a second hand FA series f1.4 for less than £200 on eBay, in perfect condition. Great lens, but why not go Sigma or Tamron? Some excellent lenses in their ranges and the Pentax/Sony fit are often significantly cheaper because they have no IS motors.
*PS forgot to say, I'm waiting for DxO's test on the D7000 with interest. I expect that to be really excellent too.