How much you'll pay for international roaming depends on whether you want to make voice calls only or also use txt and data. It's less if you sign up for AT&T international plans. But.
From past experience with AT&T, I'd say that if you sign up for these monthly charges, you may have great difficulty having them cancelled when you return. (They will continue to charge, month after month, even after they've told you it's taken care of. There will always be some reason why you need to pay for nothing for "just one more billing cycle.")
If it's "just for emergencies," or a rare call from home, just take the non-plan roaming charge hit. And use data only from WiFi hotspots. (e.g. don't use Google Maps. Use MotionX Apps, and pre-load the maps you'll need over WiFi.)
About the power, all Apple power supplies can be plugged in to pretty much any standard power grid, anywhere. You just need a plug adapter. (Not a transformer.) This is NOT generally true of other gadgets and peripherals. Sometimes, it turns out to be very difficult to explain the difference between a plug adapter and a transformer to people who should know better. ( Also, if you're gong to have a rental car, you can obviously use the same cigarette-lighter adapter from home. )