Living in Austin and working as an arcade tech for home-owners, I see this often in Texas.
I don't usually see a lot of games break down just because they are in a garage. If there is a lot of heat, you can open the garage for a little while to ventilate some cooler air, or get a window unit to help cool it down before playing. Usually, if the room isn't too hot for you to hang out in, it shouldn't bother the game too much. Since you seem to be planning on building your games yourself, just plan to install several silent-running computer fans in the cabs to keep things cool. Also put one near the top-back of the cabinet BLOWING OUT to help suck out warm air.
Of greater concern is making sure the garage is well sealed against the elements and nature. I've seen more games have problems due to water (both rain run-off and condensation/dew) and pests (mud daubers, ants, geckos) than anything system-related. If you plan on parking a car in the garage, you may also have to worry about cleaning your games more often due to exhaust residue. Also, make sure to plug all games into surge protectors to protect power supplies and sensitive parts from burning out.