I've run several successful tanks in my garage without issue over the years. You have to have a chiller, there's really no way around it.
I ran a system for a long time that had a 120g tank in the house plumbed to another 120g or so worth of frag tanks in the garage. It originally had a 1/3hp chiller but I upgraded to 1/2 to handle it more efficiently. I ended up taking that system down just because of workload at my job and phasing out of the selling/fragging side of the hobby. It ran perfectly fine though for several years.
I currently have a 30g tank in the garage just as a place to put extra coral I don't have room for and as a fish quarantine when needed. I have a 1/4hp chiller on it and it hardly ever runs. I estimate I'm spending about $20/mo in electricity to have that spare garage tank and it stays perfectly even at 80*. Trying to run it much cooler might not make sense.
Hope that helps,
Ed