In my experience it is easier to warm up a cool tank than cool down one that is running hot. The garage thing might be iffy in the summer - I generally never have to use a heater here in texas - it turns on once or twice a year maybe.

You will hear a lot of debate about temperatures but from personal experience I will say higher temps = faster growth on corals, increased metabolism on fish but also less room for error . cooler temps = much nicer colors especially on SPS and a little more room for error and a bit more dissolved oxygen capacity for fishies. A lot of people reccomend 80 - I would say 77 - 78 is about right but opinions will differ - some corals will thrive in cooler water, some in warmer. Below 75 you are getting into subtropical territory. I'd check the temperature at several times during the day and night over a few weeks to see how it's faring - also use more than one thermometer or get an accurate digital one so you can get a mean of the results . I've had the stick on thermal sensitive ones read 2-4 degrees off a standard mercury thermometer - probably due to thick glass , ambient room temp etc.