You had asked about the Pentax K20, rather than the K10. The K20 isn't yet for sale yet. The sample pictures and specsI've seen make it look like a very desirable camera (I'm thinking about it, though I shouldn't be as I already have the K10 and the K100 and have a very limited budget) but it is using a new sensor made by Samsung. It's not the same one that's used on the K10 or a Nikon camera (just Samsung's own new camera that will be very similar to the K20). The early reviews have been very positive,but until it gets in the hands of some of us amateurs, it'll be a bit of an unknown item.
As far as lenses go - you aren't limited to just Pentax lenses - both Sigma and Tamron make lenses in Pentax mounts, as well as other companies. Both Sigma and Tamron have announced that they are going to be producing some fast longer lenses in Pentax mount this year, which means that there will be more choices in lenses, through 300mm (up until this year that was Pentax's greatest limitation - the few top quality fast telephotos on the used market were bringing in very high prices).
At first I was using a Kiron lens with my Pentax dSLR, along with three lenses I bought new in 1980 - Pentax dSLR cameras can use any lens ever made by Pentax - though old M42 screw mount lenses require an adaptor. Any lens will retain whatever capability it had when it was made - if it was an auto focus lens when it was made, it will be an auto focus lens on any of Pentax's dSLR cameras. Same with exposure - if the lens was an auto exposure lens when it was made, it will retain that capability with a dSLR. However, a manual lens (like my 1980 lenses) won't suddenly become auto focus/auto exposure. This backwards capability means that you aren't limited to modern lenses though you do need to do some research and make sure youknow what you are buying.
I was going to write about the fact that the K20 will be a heavy camera - it's going to be within an ounce (I think I read) of the K10 and I think the K10 is as heavy a camera as I could manage. However, it's not as heavy as the D200, 715g (K20)vs. 830g (D200). Best thing to do is handle all of the cameras you are interested in to see which one feels more comfortable to you.