Wait for the K-3 or whatever might be in the pipeline. The K-5 is still an extremely capable camera, but the replacement seems to be just around the corner, therefore the price...
If you can't wait, go for the glass IMO.
If you don't need the sealing, don't go for the 16-50. Both the sigma and tamron are optically superior to the pentax and cost way less, at least according to the comparison on the site. The sigma has HSM focusing if you want it, but it's not worth it performance-wise, it seems.
And, as civiletti said, look at the
Sigma 8-16.