I'll second the choice of the 16-45. Here's one of my favorite shots taken with it, so far
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As to whether you should go for the K10 or 20, one thing to take into consideration is how long you plan on keeping it before upgrading. If you're thinking you might want to eventually upgrade to the K20's successor, then get the K10; when you go to sell it, you won't lose as much money as you would if you had bought a new K20. OTOH, if you think you'll be keeping your body for a good while, then go for the K20.
As of late, I've seen several threads where K10 owners are asking whether or not it's worth it to upgrade to the K20, so you might want to look around for those and use the info to come to your own conclusion.
HTH,
Heather