@Guy, I suggest sending message when the freezer is above temperature not below.
If only sending when below it means you need human logic on the receiving end to differentiate between loss of communications and an actual failure. If instead you make the normal situation send a message; powered and below set point, it is easier to handle.
GPRS is doable, but internet is quicker and easier these days, and just as resilient. (i.e. GPRS isn't intended to be resilient). If the site has a wireless network, and the IT guys are sufficiently welcoming, then use it, or add your own wireless router just for the job.
I suggest an Adafruit Huzzah for each freezer which wakes up every minute to send temperature readings of inside cabinet and outside heat exchanger.
Power the device from within the appliance by adding a power supply module in a cavity.
Handle the infrequent deliberate defrosting by just ignoring it. Adding that into a monitoring system makes the system way more complicated and needs extra training for the people using the freezers. A much harder task.
If you need a display at each freezer, a MicroView could be good.