Question: How big is your garage and is it attached to the house? I just started brewing in our attached two car garage, about 25 x 25 x 12, with a 10" propane burner and do not need any extra ventilation. Hot water is brought from the kitchen sink in a 2.5 gal sanitized water jug, a few trips required but much cheaper than plumbing and heating the garage.
Brewing waste water, always a very small amount, is just dumped in the back yard. The cooling waste water, 5 gal from MT (I pump ice water from the 10 gal MT, through an immersion cooler and back to the MT) is likewise just dumped in the back yard. The brew table is a piece of 3/4" plywood, 6 feet long and 30" deep, hinged to the wall with fold down legs so it can be folded against the wall when not in use.
Having an electrician for a friend is great. He can help you layout the brewing area. My electrician is very busy. He told me what I needed to get at HD and let me run the wires and install the outlets and switches, the most expensive part if the job, then he inspected the work, made the final connection to the sub panel and signed off on the job. If your friend is too busy to do all the work this may be an option for you.
I used fiberglass insulation as, like your garage, there were no interior wall coverings. We then used 3/8" plywood to cover the walls. Didn't want to constantly deal with broken sheetrock walls.
I heat the garage with a kerosene heater. It takes the edge off but don't expect to be able to brew in a T shirt. If you have a single car garage, which it looks like you do, and plan to heat it with a kerosene heater and brew with a propane burner, you may need some ventilation as others have suggested. Leaving a door or window open a but should work. A CO2 meter may add another level of safety.
Hope this helps.