Thanks for all the feedback folks
I went ahead and ordered the kit including the 18-135. In the end, what won me over was realizing - as the previous post underscores - that the Sigma C doesn't seem to be
significantly better (to me, with my priorities and sensibilities, that is), while I would lose out on weather sealing. While I've done okay without weather sealing until now (though I'm not sure what killed my old zoom, I kind of doubt moisture was the culprit), going to Cambodia and Vietnam in the rainy season with a brand-new, somewhat expensive, non-sealed lens seems ... risky. Not getting something wider than 18mm is still a bit of a bummer, but one that I can live with considering that I have the 10-24 for when I want more. I also got a spare battery (duh) and the O-ME53 magnifying eyepiece - not sure if I'll end up keeping that, but I'm thinking it might come in handy for checking focus with the 10-24. Might be redundant with the actually useful live view and (tilting, non-tiny, non-awful) LCD on the K-70, but that's not a luxury I'm used to, so I suppose we'll see. No doubt everything will be a massive improvement from my K-x, no matter what.
Now I'll just have to wait for the camera to arrive. It'll probably be a while - I'm expecting a week and a half or more - as no stores actually had it in stock (consequences of buying a niche brand like Pentax in a tiny country like Norway, I suppose). Still, it'll be exciting once I have it all in hand! Definitely looking forward to trying out the new lens too, of course