I don't live in Florida, but I know what the climate is like there. I agree totally, if you are going to build the layout in the garage I would seriously think about making it climate controlled. First of all, it will make it a lot more comfortable for you, a lot more pleasant experience.
More importantly, keeping the temperature relatively constant is better for almost anything on the layout, the wood, the scenery, and of course the track and rolling stock. The biggie besides temperature is humidity and that can cause damage, especially if you are using steel track (in theory nickle silver like Atlas uses would not have that problem). Given you are custom building a house it would make sense to address it at this point, would be a lot less work than doing it after the fact. A dehumidifier might be necessary if the hvac system that is there doesn't keep humidity low enough in the humid months.
Central Florida can also get cold in winter as well. I don't know what they are putting in the house, will they be putting a heat pump system in for the HVAC? Or is it just central air? Is there any kind of auxiliary heating going to be in the house? If it isn't practical to extend a main system to the garage, you could get a mini split system installed, for the kind of temps you are talking summer or winter it likely would work great in there. When you aren't using it, you could prob set it for 55 in winter and like 78 in summer, and turn it up/down when you are in the garage and the layout will be fine.
Just my thoughts,
