I have a GC in my garage. As for AC, if you have any vents going to the outside (like maybe for the dryer, if it's in the garage) you can run the ducting for a portable AC unit to the vent. You can't overdo it with the BTU's. An insulated door would be a big help for climate control. The only advantage to a non-insulated door would be if you need the extra inch or two of lengthwise clearance the thickness of the door would give you. Even some well placed box fans will help with air circulation but you want to experiment with their placement to avoid wind over playing surface causing roll-off. On the floor below the playing surface level, for example.
A couple of other considerations:
Arrange the table so you are racking the balls towards the back wall end of the garage, not towards the garage door (head of the table where you break from by the garage door and foot end towards the back wall). This way if you have the garage door open or cracked and a ball jumps the table when you break, it is less likely to go bouncing on the concrete or asphalt outside and get scraped up or chipped (plus now you have to go chase it). If you have windows along the top panel of the garage door you will probably play with the door cracked during the day unless you put shades over the windows or black them out just so the light isn't streaming in. Also, to protect balls that may come off the table as well as to afford at least a little protection in the event of a dropped cue, I'd advise laying down some indoor/outdoor carpeting.
If length-wise clearance is an issue (probably not with the 7' table, but perhaps with a 9') and you have to cheat a little in one direction, give yourself the extra bit of room towards the back wall / foot end. Percentage-wise I would think that more often the cue ball will be on the foot rail rather than on the head rail so if you have a length of the table shot like this you want to have enough clearance from the back wall. You don't need the full clearance at the head end for breaking because no one I know puts the cue ball all the way back against the head rail when snapping. Additionally, if necessary you can always temporarily open the garage door to execute a shot but the back wall would be a permanent obstruction.
Consider off-center placement of the table in the garage. Depending on how wide the garage is, don't forget the possibility of being able to park a car in the garage. My garage is 20 feet wide. I put the table not in the middle, but rather a little over 5 feet from one side wall (more than the minimum clearance). My wife still has ample room to park her car in there (concession to the wifey). Even if I was single I would prefer to be able to park my own car in the garage. I just take the car out when I use the table, no biggie. If you can do this, the table is probably better off on the right where the driver's door won't be banging into the table every time it is opened. Even if you don't have room to get a car in, another advantage to this off-center placement of the table is that you won't have any issues with hanging the table light and the garage door opener being in the way. My opener hangs from the center of the garage ceiling and would have caused an interference problem hanging the light if the table was in the middle. Lastly, off-center placement gives you more storage area on the one side so whatever stuff you are storing out there (boxes, bikes, etc. if not parking a car) will not be in the way.
Remember with measuring clearance, the key measurement is from the cushion nose, not the outside of the table. Also, if you use more than a standard 58" cue you need to adjust your measurements.
Back on the garage door, a roll-up door is better than the tilt up kind. This is because a roll-up door seals the garage when it is down. It keeps the garage (and table) cleaner for one thing. Secondly, when you use the table at night a sealed door keeps the bugs out that are attracted by the table lighting. Bugs can get in through the cracks around and under a tilt up door and they are a pain in the ass (buzzing around under the light or landing on the table) when you are trying to play.
Buy a good, heavy duty cover since garages just don't stay as clean as the inside of the house. Pooldawg has good ones that are cheap (around $50). Make sure it becomes a rule of your house that NOTHING is to be put on the table - ever. My wife knows not to put the groceries on it when she takes them out of the car, not even for a minute. It is not even used to fold laundry, even with the cover on. For storage there is plenty of space under the table that can be used that won't interfere with a player's stance.
Hope this helps. If I think of anything else, I'll post it up.