Thank You all, very much for all the infos! 
A few additional thoughts:
- Yes, cheap prices would be nice, because in my country nobody would pay $200+ for a device.
- But sadly I do not have much time and all the equipments to solder and box everything together 10-20-100 times.
- Also most of those 5-10+ fridges are far away from each other, so it would be hard to daisy chain all of them together!
- In most cases even wiring + drilling wholes is complicated for most of the restaurant owners, and they will not pay me enough to travel 50-800Km to do it for them.
(Not to mention they are open every each day, from ca 07AM - to 23:30PM … so any drilling + wiring is impossible in that time while people are cooking food there!)
So I’m looking for some kind of “golden middle way”.
