Being from Chicago... I know that... "it do get cold in the winter time!" It seemed that the ground could get as cold and frozen as it is in the Tundra.
I have several low-voltage lighting systems... out here in balmy Oregon (It rains alot, but it doesn't get nearly as cold... the rainy days make every sunny day a delight!) Condensation is a way of life out here.
I wouldn't worry about the metallic, electrical components.
I should imagine that the primary concern, in your neighborhood, would be the ability of the plastics in the transformer unit to handle the rate of change in the temperature in the unit when the unit goes from OFF to ON. The same might be true for the plastic in the light fixtures.
A lot of plastics don't handle rapid temperature changes very well. If a particular transformer is heavily loaded then I should expect that the ambient temperature (inside the transformer unit) would change rapidly and greatly. The plastic might not be able to handle the change without cracking. That could cause problems later on.
I remember, back in Chicago, being victimized by the "Detroit-Plastics" in my car - many times. Back then, "Old-Man Time" and "Jack Frost" would make a plastic fitting brittle. At some point, applying a current to the conductors, through the connector, would break the plastic. In most cases the connection would be maintained but then the connection would be exposed... corrosion would develop and the connection would go to hell.
Sometimes, during a "Spring Tune-Up", simply looking at a connector would cause it to shatter. At the very least, the locking mechanism would come away from the connector (broken) as I tried to open it.
Even if the plastic can normally handle the temperature change, I should imagine that "Old-Man Time" will take his toll.
Maybe the thing to do would be to run a switched heater-circuit to each of your exposed transformers (easier said than done, huh?). Break open the transformer enclosure and install a low-ohms, high-wattage resistor somewhere in the guts. I should imagine that 5 or 10 watts of heat would be plenty (something on the order of a small Christmas tree light). It would be better to use a resistor than a light bulb... filaments tend to die... wire-wound, or carbon resistors that are under-driven, will last forever.
In the winter time simply leave the heater-ciruit energized. This will mediate any radical changes in temperature as well as prevent any condensation issues. It's not like Wisconsin suffers from high humidity problems in the winter time.
In the warmer months leave the heater-circuit off.
You might even have the heater-circuit controlled by outside temperature. No sense heating the transformer when you don't need to do so.
If you have metal light fixtures then I wouldn't worry about them. If they are plastic, then you might want to use bulbs with lower wattage.
Basically, it's gonna be a try-n-buy situation.
Congrat's on the new house!
I surely do miss the weather of a good ol' Midwest, Winter Season! (now and then.)
One of the things I remember most about a Chicago winter was that you could be up to your arse in snow with frozen boogers hanging off the tip of your nose, but it would be OK because the sun was shining!
Of course... that was before I had to drive through the stuff to get to work!