Welcome to the forum. Like Tonib I have no experience of pet monitors.
Nice idea with the equipment and I can see why some people would like it! Depends totally why you think you need it, are you wanting to leave dog in car at times of year you should not ....... Is it just to make sure on a dull, dank, overcast and cool winters day that dog is actually safe from overheating?
At same time I can see it as an excuse to try and leave dogs in cars when weather is not suitable to do so (i.e any time from mid spring to mid autumn or even on a sunny day in winter), on the premise that the equipment will warn owner if temperatures rise. Not good idea at all! What if there is no mobile signal or wireless signal (common where I live)the warning I suspect can not be sent!
I can see pros and cons with the monitor but more cons than pros as people will use it as an excuse to leave dogs in the car when they should not be risking it!