If you're really going to be using it that infrequently, prepaid is the way to go. It's far cheaper than any monthly plan for really low usage customers like you (and me).

If the fact of the matter is that you're never going to use any feature of the phone except answering and dialing, any phone will do. It might have a thousand features you'll never use, but who cares? There's nothing that will prevent you from ignoring them, and nothing that will get in your way of just answering and dialing. AFAIK, no phone hides answering or dialing in any menu at all.

Oh, and usually simple phones are cheap enough that it'd be easier to just get a new phone when you move to Mexico.

If you want to be able to keep the same number, I'd suggest getting a number from a forwarding company and just have that number forwarded to whatever cell phone number you happen to have at the time. Heck, get a VoIP service that will ring your home and cell phone at the same time.