As long as you use imessaging while connected to WiFi you won't get charged a thing. To be 100% sure, remove your SIM card before getting off the plane.
I would think you would not be charged international rates if sent via wifi. I agree with RushFan2112, though, to take out your SIM card to ensure you are not using cellular to send/receive.
Additionally, I disagree about SMS/MMS not being able to be sent/received via wifi. I have a wifi only tablet and an unused(i.e. no SIM card) LG G3 which I have Verizon Messages installed on. BOTH of these devices can send/receive SMS messages even though neither one has cellular capability. I can't remember if the LG G3 has received any MMS messages, though. You may want to install/activate Verizon Messages on your device before leaving on your trip.