Temperature swings can be an issue. Expansion/contraction cycles can cause damage to circuit boards, components, etc. Ideally you want to keep the temperature pretty stable.
The 12 volt power supply is going to generate some heat. Might want to use that to your advantage. Putting the repeater in an enclosure of some type to keep the temperature relatively constant will be helpful.
Heat buildup in the warmer months can be an issue, especially if you think the repeater will see a lot of use. A fan to circulate air through the enclosure can help with that.
Rigging up the fan to a thermostat would be pretty easy. I've got a couple of very remote repeater sites that have a 10 watt UHF repeater in a box run off solar and backup batteries. The enclosure has a small 12 volt fan and a thermostat to turn it on if it gets too warm. Air temps rarely drop below freezing, so it's own heat generation keeps it stable in the colder months.