Armanius, I realize it's been months since you posted the above... but how has the Sigma 18-50 f/2.8-4.5 worked out on the K-5? I've got a DA* 16-50 f/2.8, which is big and heavy, and a DA 16-45 f/4, which isn't bad. But I'm always on the lookout for lighter, high-quality zooms in that general range.I'd have preferred less zoom but faster lens, if a trade off is needed. I'd be happier with a 18-50/f2.8-3.5.
I recently picked up a cheapo Sigma 18-50/2.8-4.5 for the K5. Slightly larger than the kit 18-55 and the DA18-135. But internal zoom, HSM motor, and OIS. That internal zoom thing is awesome.