help with garage growroom

Hi,
I have an enourmous garage.
right now my grow is in the basement, which is not the best place for a variety of reasons..

So, I have this idea to build a room in the garage. it has 10 foot ceilings, so I can grow some big sativas.
My biggest concern is how to heat/cool the space. I live in a northern state so it gets hotter than hell in the summer and freezing in the winter.
I am just a grower for personal use so the space doesen't need to be huge, no SOG or perpetual harvest. I want to light it with a 600 HPS/MH. That should keep it warm in the winter but what about keeping the temps down in the summer? I don't want to have a conspicous a/c unit sticking out of the garage, plus it would probably make it to cold in the growroom.

any thoughts?

One 600w light is not enough to really do a garage grow. You need enough light to keep it warm enough year round and one 600 won't cut it. If you had a couple or a few on alternating 12/12 so one is always on then that would work. Otherwise you'll be burning more electricity to heat that space with a heater than you will be growing with. Even a 1500w heater can't cut it, twice the heat your lights can put out.

After you have more lights, then you have a thermostatically contolled exhaust fan, big, and that should do it in all but the hottest months/climates.

If you end up needing a/c just to cool the garage in summer without lights then jesus, your light won't be even 1/4 of your heating and cooling costs for that garage.

If you must use the garage you should create a very well insulated box to grow in and only heat and cool that.