2.5G refers to the data network that iPhone (the first one) works with. The new one works with both 2.5 and 3G.
The old one (with the metal back) is the one that @Eambos was referring to. There are plenty of unlock guides for these online.
Be sure that you go with a carrier that is GSM, as ATT is GSM. T-Moble, is another that is GSM. Verizon will not work with iPhone.
Go with caution, however. If your mom is the one buying, she would have to sign a contract for the phone. If she truly intends to buy the phone, she will be more-or-less forced to get on a contract at the time of purchase, in which case you would not need to unlock, because someone would be paying the bill on an ATT contract.
If it was me, knowing what I know as you said in the question, I would wait until after my birthday before actively pursuing an unlocked iPhone. You must also weigh the task of maintaining and researching the effects of software updates (that can (nearly permanently) render your phone useless) against the cost of having a phone that works well, albeit on a carrier that you don’t like.