It all depends upon your budget. If you can afford it, I would suggest the K7 within a couple weeks after its replacement is announced probably in September. The K7 since it is very similar to your *istD - two wheels (front and back), and your transition should be pretty smooth to the new body.
If not the K7 then possibly the K20 - 2 wheels again, and then probably the Kx which has a single wheel. Another thought is that there will probably quite a few K7s and K20s coming on the market place with the K7 replacement, that could be had at a very good price.
I have had very good luck with my 18-55 version I on my K20. I remember reading that the original kit lens was not up to the higher resolution of the K20. I do still like my 16-45, but the kit lens shoots very well too. In terms of glass, it really depends on what type of photography suits you best.
I upgraded last year at the K7 announcement in the same fashion, and the K20 and I will be going forward until it wears out.