Actually I would skip the data plan altogether regardless of carrier. My current needs are meet with a Nokia 2115i and a prepaid plan, so I would do similar with the iPhone. The gain for me would be the apps and wifi abilities - yup I know I'd be giving up a lot of functionality, but it is functionality that I currently don't have and I think I'd rarely use.
Of course this master plan all depends on finding an iPhone for cheap...
Well then I guess you'll be fine! The only problem is finding an iPhone unlocked. I doubt you'll find a 4 for cheap, but considering AT&T is now offering the 3GS for $50 (on contract, of course), you might be able to find that model for less money than you used to.