If you're going to undergo the effort of writing an app, it seems that the cost of getting a Mac Mini is peanuts compared to that.
If I had the money, honestly, I'd do it, but unfortunately I don't.
What else do I need to do iPhone dev? Won't I need some sort of actual device aside from an emulator?
Mac mini's are great, or pick up an old model macbook. I would also highly recommend the book "Cocoa Programming for Mac OS X".