Iphone data help

If I were to buy an Iphone off of eBay, could I buy a unlimited data plan from AT&T or would I have to go through the process of changing the plan. I would probably get the original iphone, but I am on a family talk plan and I would be the only one with an iphone. Could someone tell me the best way to go about doing this? and about how expensive it would be? Thanks

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you can still be on a family plan, all you have to do is hook the iphone up to itunes if its the original one, it will delete your voicemail and turn on the visual voicemail feature but you'll still have to re-set up your voicemail box. it will then put you with the $20 iphone plan also. the new iphone you can just call in and update the imei but with the original you have to activate it through itunes. it doesnt make a difference if you are the only one with an iphone its fine, you'll just have a $20 charge extra on your line.