My water change system was brutes on wheels before my new setup
I'd recommend ~25g every couple weeks. That would be ~20% per month. That's plenty as long as you have good, skimming and don't overfeed. Plus you will get a feel for it as the tank matures and you can tailor your setup to match. Maybe you'd rather do water changes than heavily dosing your system with chemicals, then you will require more water changes on a heavy stocked reef system. If you don't mind dosing the required chemicals and have good nutrient export through either skimming, algae turf scrubber, bio-pellets, etc. then you can really stretch out the required water changes. The triton method of testing suggests never changing your water again unless you can't export the nutrients and adding the chemicals and trace elements the inhabitants are consuming.
As for the garage idea, sounds iffy. You may be wasting a lot of power just trying to keep the water warm enough. No room under the stairs or in a closet in the house?