I ran into the same situation a few years ago when I was officially "banned from the Galley" Turned out it was one of the best things that could happen. First, a few things to ponder:
Question: Is your garage attached or separate from the house?
Why: If the garage is separate, you should invest in some good food safe grade water hoses to get water into the garage... a good place to look is an RV parts sales store. Conversely, if the garage is attached and heated, consider adding plumbing and a deep sink.
Question: Do you have proper ventilation in your garage?
Why: Most commercially available turkey fryers or LP burners used to heat your kettle or MLT can produce a significant amount of CO gasses. Just cracking the garage door open a bit may not be adequate to keep you safe.
Question: Is "your half" of the garage going to be used for parking a vehicle when not brewing?
Why: Good clean and organized storage of your equipment IMHO is a must. A place for everything and everything in it's place will only make your brewday go more smooth.
Question: Is your garage heated?
Why: I keep my garage at about 45 deg F. during the winter. At that temperature, I found I can still ferment ales with a little ingenuity and a trip to the hardware store. see the bare-bones-fermentation-cabinet
. Also, if the garage is heated, you may have the opportunity to design your brew rig for use with natural gas... check out Nicksteck's Rig
Some items that have made a big difference on brewdays:
Kegerator (Gotta have home brew handy)
Squeegee - the floor is going to get wet at some point... having a good squeegee is nice for quick clean up.
Water Filter - I've got one conveniently mounted inside the door ( I use hoses to get water to the garage) I just hook up the hose and filter the water as it comes in.
Hammock, blankets and a pillow - let's face it, she kicked you out of the kitchen, now you wont be spending time with her in the house.... could be a few rough nights ahead.... but she'll get used to it and it will get better! Besides, it's always nice to have a quick fix for an intoxicated friend who came over for a brewday... a night in the hammock is better than a night in the clink.
Take your time and plan well..... then build slowly as things will change.