engine:
I've played through all or part of KotS more times than I can remember, so I'd rather not play that, if there's any other option.
I can make a couple suggestions.
It has some flaws, but the Scales of War Adventure Path would give you plenty of material to take us all the way to level 30.
If you want something a bit more freeform, it's worth digging up the Chaos Scar adventures. It never quite achieved the goals they had originally conceived for it, but it had some interesting elements once you had everything together.
And there's always simply starting out in a setting with an introductory adventure to get us together, then just see where our own decisions lead us. You have no shortage of supported settings — the Forgotten Realms, Eberron, Dark Sun, or something homebrewed. I'm a big fan of Eberron myself, and it got decent 4E support — it actually handled some aspects of the setting better than the original 3.5 version.