Buying a new body won't take care of your need/want of a walkaround lens. IMO, you're gonna get more bang for your buck from a lens purchase than a body upgrade.
If you want the best superzoom, I'd pick up a Pentax DA 18-250 on the used market. It's around $250 used, and widely regarded as the best overall walkaround superzoom. It's optically better than the 18-135, and covers more tele range. The 18-270 is also well-regarded, but a lot more money.
Personally, I use
this Tamron 18-200 for a walkaround, and I'm happy with it. I've used a few superzooms, and while any superzoom is a compromise, this one is pretty good. (or at least my copy is.) I've gotten some good shots with it. You can pick one up for around $125. It may not be as good as the Pentax DA 18-250, but for me, it's good enough.
A couple taken with an *ist DS & the 18-200 Tamron:
Good luck & have fun!
Bob :-)