To the people who say the 16-45mm is not up to the K-3 sensor - are you talking from experience? Because I use it on my K-S1 (20MP) and it's fine. Sharp enough. The extreme corners aren't up for pixel peeping but that's usually true of any zoom, I'm pretty sure the 18-135mm won't do better.
The body suggestion would depend on budget, but if the budget is there for a K-3, that's probably the best bet - unless low light performance is needed and the budget is there for the KP, which does have a significant improvement over the K-3 in that area.
You might as well buy a K200D or K10D on the side, to use just in case you miss the wonderful CCD sensor colors
I bought a K10D last year and I use it more than my K-S1...