What hardware do you need to buy in order to publish apps to the iTunes/iOS App Store?
Technically, the submission of apps and music and books happens over a web page, so you don't need any Mac hardware to publish the end results of your creative work.

For iOS, the Xcode tools run only on Macs, so you simply need to pick a version of Xcode and find a mac that meets the minimum specs. Pretty much any mac made in the last 5 years is good enough. Do make sure the mac will run Lion as you won't want to be stuck on Xcode 3 for long. You do not need to have any iOS devices to submit an app to the App store.

However, having iOS devices is highly encouraged for testing and development, but you don't need one for publication. Testing on actual devices is very important for a quality app as there's no replacement for getting the gestures and the UI tweaked on a real device. Plus, the simulator isn't quite a perfect simulation, especially when it comes to memory, so you may miss bugs you would otherwise find on an iOS device. Apple will surely run your app on several devices and you will lose time and perhaps not be approved if you ship apps that run fine in the simulator, but not so well on real devices.

The simulator runs Intel Processor code, the devices run ARM code - so your one code project gets built into two apps - you only submit the iOS app for approval / distribution and it's better for you to test that exact build and find the bugs yourself rather then when the app is under review.