We live in Minnesota, and just got thru a very cold week, with low temperatures in the -20 range.
No problem- just to be safe, we upped the time for our filtration cycles, probably didn't need to, but did it for peace of mind.
If you are away a lot, maybe get someone to check in every few days, just to be sure the power is still on.
Hate to have a breaker trip during a cold spell.
We use a bromine routine, and have no water quality issues when our spa goes unused for a week or so (only when it's really cold and windy).
Not sure about your iron question, since our tap water is pretty good. Take a sample of your outside tap water to your dealer, they should be able to offer suggestions. They may have other customers with in your area with well water.
Brewman