Getting a little confused with this thread...
The typical deep freezer thermostat will not control at the temperatures you want. They are designed for lower temps. The cooling system itself will do whatever it is told so it will work if the thermostat works in the desired range. What the appliance guy you spoke to may have been saying is that the thermostat that comes with the unit cannot regulate at the higher temperatures you desire. Even setting the thermostat at its warmest, the temperature will get much cooler than you want before it ever kicks off. You will need an after market temperature regulator as mentioned.
Once again confused...
I am also worried that the freezer is not used to being un-plugged then re-plugged in and that this may cause the freezer to fail entirely in a short period of time? I'm sure that the built in on and off system within the freezer is less harsh then the described mechanism?
The compressor is on / off... no matter if it is un-plugged and plugged in to maintain temperature or if the supplied thermostat controls it or if you use an aftermarket thermostat to control it, it's just on / off. What WILL shorten the life is if this cycling occurs too rapidly (short cycling). When the compressor kicks off, the system is at a high pressure from running. If you immediately try to turn the compressor back on, it is now working against this high pressure and the starting current of the compressor is very high and will shorten its life considerably. As long is the compressor is off for a length of time (say 10 mins) before it is turned back on, it is fine. During this time while the compressor is off, the system pressures equalize and when the compressor tries to start, it is starting against a much lower pressure and will not draw the high currents as mentioned before.
Until you get into upper end expensive home air conditioning units and commercial units, you will not have any pressure regulating / equalizing lines which are the solution to this problem.
In summary, get an aftermarket thermostat!