iWork only runs on Apple's hw, but I didn't think you needed to buy _new_ hw. I believe you only need to update to Mavericks (or Yosemite).
They changed it with the release of Yosemite. It's now free for anyone with iOS8 or OSX 10.10
I don't think so. I have an older MacBook Pro, and I'm running the latest OS X (and iOS). Comes up as 20$ each for Keynote / Numbers / Pages on OS X, 10$ each on iOS.
Nope. You need to purchase new (late 2013 or newer) hardware to get it.