If I was in your situation I would be looking to get a phone with very clear and easy functionality. There are some truely awful and frustrating to use phones out there. As far as I am concerned the old Nokia interface was by far the clearest there was. Unfortunately you can't say the same for their current phones.

So I'd look to get one of their few remaining phones that uses the old UI. For example the Noika 1200. It isn't listed on the US site, but it is on the UK one:

http://www.nokia.co.uk/A4405747

In the UK it is available very cheaply as a handset only without contract, for example only £22 here:

http://www.theorder.co.uk/simfree/offline/deals/Nokia/1200

As well as the old basic UI it also has the benefit of a monochrome LCD screen, which are still far more readable in all conditions that colour screens. They also tend to be more robust than more recent phones, we still have 5 old style Nokias hanging around the house somewhere that still worked fine when we stopped using them. All the more recent phones I have bought have died within a couple of years.

Oh and it has a standby time of 390 hours, that's 16 days

Edit: not sure if it is suitable for the US/Mexico though, can't remember what GSM frequencies are used where, it does "EGSM 900/1800 and GSM 850/1900"