I can't comment on which IP# to get, just wanted to say if you're
specifically looking to buy a phone that you want unlocked and usable later - register for an account (free) at imei.info. You can check if the IMEI is carrier locked, sim locked, or blacklisted without having to pay. The status updates occur very quickly, my phone showed unlocked within minutes... FYI, in case somebody tries to pull the wool over ya. And all IP's have water damage indicators, just google it up for which version you're looking at and there you go - no need to fuss with the "genius" bar.
And although it never hurts to have spare phone lying around, if you think it'll really be at least a year or two before you use it then I'd probably suggest just getting an ipod touch. More bang for the buck
