This camera is excellent, and can certainly handle low light by setting the sensitivity to higher ISO's (or through the use of faster lenses). However, like most cameras and sensors, the use of higher ISO's will introduce "graininess" in the images. Sort of analogous to higher speed films in the old film days. Photographing documents is possible with most any camera, however, the quality of the images are dependent on the quality of the lenses. Depending on the document to be recorded, it may be necessary to invest in a macro lens. In essence, the camera body itself has little impact on the ability to photograph documents. Hope this is helpful.
ePHOTOzine.com just published their full review of the K-5. Looks as if real-world performance matches the expectations raised by DXO. And the rest of the camera performs at that level.
Hoya has succeeded in bringing Pentax along very nicely - this model competes very well indeed in the $1,000 to $2,000 price range. Canon and Nikon will have to work harder now and that competition is good for all of us.
Yes. Any K mount lens should work, with various restrictions. All M42 screw mounts will work with an adapter. Depending on the age and feature set of the lens you might need to use manual aperture, focus, or metering, though.
The prices of various sellers on Amazon defy common sense. A camera strap that the manufacturer lists at $29 is sold by Amazon for over $60; another Amazon seller has the $29 price, "marked down" from $169!! Other camera items (including cameras and lenses) that sell new on Amazon at one price are listed used on Amazon for hundreds of dollars more! This is true in all product areas. For example, I have a surf book on Amazon, new, selling for $19.95. The same book is listed (right now, today) used at $58.88! There is no rhyme or reason to the pricing.