CNet has a list of basic phones that they keep updated. They key thing for international travel is indeed GSM, though Mexico does also have CDMA providers (the other major standard in the US). AT&T or T-Mobile both offer GSM service in the US. And this site lists the Mexican GSM and CDMA providers.

And I agree with Tony, decide on who offers the coverage you need, then look for a plan (prepaid/monthly, etc), then get the phone. Most electronics retailers also now carry prepaid phone kits where you get the phone, and first $20 of service in a box. Take it home, turn on the phone, and make a call to set up the account.

With the way rates are, I'd recommend going with a US provider and a US number if you are going to be here for another year, then look for a provider and service in Mexico when you get there. Keeping a US provider long term in Mexico is going to be very expensive.