I disagree with the Pentax statement. Matter of fact the Pentax will mount after market lenses. Including Sigma and Tamron lenses! So there are lots of after market lenses available for the Pentax system! And as for used lenses, unlike Canon. Pentax is like Nikon and can use their older lenses. Heck you can even mount their MF lenses on the darn thing. Since you can use screw mounts you can now add Zeiss to the list of lenses you can use on it as well.
Well said. I've just got myself a second had 28mm lens for my Pentax DSLR.. and it is a 30 yr old M42 mount lens. Lovely lens and very cheap.
Fact it's manual focus isn't a big problem and if someone is using a lot of MF lenses on a Pentax then you can quickly, easilly, and fairly cheaply swap out the matte screen for one with a split centre to aid with manual focusing.
Regarding the K20D.. it is THE BEST sub professional DSLR on the market at the moment. There are some with better features such as faster 'motor drive' (only really a POSSIBLE issue for motorsports work) and better Live View ( very few people use that) but pound for pound the K20D is unbeatable. Well built, very very keenly priced, superb range of lenses. It is also good enough for Professional quality work especially when paired with some of the awsome, low priced, glass that is available. (that isn't to say proffessional, especially full frame, Nikons and Canon's aren't better. They are.. but they cost 10-20 times as much cash.. just for the body)