I'm not a big expert in cellphones myself, but I believe the big thing is that it be GSM and not the other type that I can't remember (CDMA? There are two big carriers in the US that still use that, I believe). There may be some frequency issues (I've seen people mention this before), but I've never looked at that to be honest - I've bought a number of unlocked GSM phones in the States over the years and they've all worked here, though never a Motorola. Looks like the model you mention has GSM support, not sure about frequencies it may operate on.
As far as pesos from the ATMs - the rates are good, blue rate is not so important these days because the prices are so close. However, specifically with ATMS, the problem now is the limit that you can get out (around 2400 pesos per transaction) and the high fees on this end (around $6 USD per transaction), plus whatever fees your bank might have. It makes individual transaction expensive still. If you have something like a Schwab account that gives back fees (even if you have to push them to do it) it might be doable. If you're here for a lot of time and can't bring enough cash with you, you can certainly use the ATMs to supplement your money but don't depend on it to provide the majority I'd say.
