I believe the issue with a self defrosting or frost free freezer being used at less than the recommended 40ยบ is related to the defrost heaters and the possibility that there could be some drainage from inside the freezer that could freeze in a drain or evaporation pan and damage something.
There is also a possibility that freezing temperatures can upset temperature controls or damage the compressor. I do not believe that is the case, but manufacturers recommendations should always be followed to the letter.
That said, what I would do is to check online for an appliance repair specialist that sells parts for the freezer that you are considering to see if you can determine whether the freezer will function OK without the self defroster being hooked up. If it can, it is simple enough to remove the wires from the defrost timer so it will not defrost during the winter.
If the contents of the freezer was valuable, I would monitor its temperature using a K probe and a meter. If you do not already have one, ThermoWorks can fix you up with one.