The Verizon iPhone 5 has its SIM/GSM portion unlocked from the factory. You will have no problem other than trying to find a nano SIM to put in it.

I have a T-Mobile UK prepaid SIM active for travel. The data roaming cost isn't too bad, but it has to be reactivated via SMS every 50MB. I also have a Roamline SIM that's data-only. I have a plethora of VoIP applications on my phone, so that's typically all I need. If you're staying in one country, you should just get a local SIM. Of course, it helps to speak the local language when dealing with customer service (refills) or setting up data packages via SMS.