Yes. At one point, my household had 3 guitars, a drum kit, and a mic, but only so multiple people could play the guitars at once. I would get the guitar that comes with World Tour, as it has the tap bar, and you're set for all games, Rock Band included. The ONLY difference between GH and Rockband is that some of the early games of GH, the RB guitars don't work, and that if you're going to get a drum kit for both games, get the GH one, as it has an extra "button" that the RB kit doesn't have.
A pricey investment, but it covers all games in the series, so it's worth it's coin.
Indeed. You at least need a guitar (some games are bundled with one). Then there are drum and vocal peripherals if you're interested (not required to play, but required for achievements in games later than Guitar Hero III and Guitar Hero: Smash Hits).