A point of view after a few decades in Houston;
* A non controlled area sitting on a concrete slab will be close to ambient, for me to get 60F I put up a foam wall shed with window AC. Do not expect 50F on the ground.
* I use my under the stairs for storage. I like the spot but for conditioned space insulating will be more work. A refrigerator has about 1.5 inch foam in place, this is good enough. Yes my sample shed was 4” but it sat outside on a slab.
* your number one need is to moderate temperature swings, a constant 75F will be better than swings from 70 at night to 90 in the day. You could reduce the effect with a tight, low mg oxygen per year bottle seal. ie Off the shelf, natural corks breath creating oxidation.
* with green or brown glass bottles one can mostly ignore light. With clear glass the UV from grocery store lights will have tasteable effects over a year.
* Your local AC vendor will have good ideas. They may not have done wine but temperature is an issue on a lot of food products.
My question is how important is this hobby? Are you willing to give up a room? In the ideal you would also run a controlled temp fermentation area. At the minimum size up only long term storage cases with something looking like a fridge.
Part of a wine cellar is display. This means 3’ aisles.
Oxidation will happen even in a freezer. Looking at how to use antioxidants like tannins and free SO2 is a good use of energy. Also head space, oxygen exposure in racking, ,,, “total package oxygen “.
