I recommend getting the inRach Mini 2:
They work very well and the 2nd generation Mini uses a different (read: better) app than the first one. To send messages you will need a phone that is connected to the Mini via Bluetooth but you obviously do not need a cellular connection as the unit uses the satellite network to send/receive messages.
And yes, depending on how much you want to spend on your plan, you can "pause" it and then resume it with no penalty. I pay about $25/month and for that amount, I can send as many messages as I need to and I can pause the service if I don't anticipate using my device for the next 30 days.
Hope that helps.
Martin