I work out in my garage now also. We are in Washington so had to insulate for winter. Put in a quality garage heater so in the winter can get it to 65 for comfortable use. During summer, another story. So stinking hot I work in 85 to 90 degrees. Yep, working on an airconditioner but would have to put in a window. Fans help. For your garage door, if the foam board does not do it for you, they have this insulation stuff that is silver at Home Depot that you put across the door and it insulates it and I can still open as needed. Works wonderful. Search for garage door insulation, you will see it.
A nice counter/cupboard space is nice to have. We found some that were being thrown away from the church which we painted and installed them. They are across one wall so I can store my threads and fabric in there so they are not damaged by the elements when the door is open. You will want a cushion pad for under your cutting area. I do longarm quilting and have truly enjoyed having the foot cushion to walk/stand on. The cement floor is rather hard on the feet and knees even if you are sitting part of the time. You will want to be sure you have enough outlets before you start putting everything in. Those are so important or be sure you have your extension cords set so you can always have access to power when moving about. Be sure your breaker box can handle the iron and sewing machine on same outlet. In my house it can't so had a couple more circuts put in with outlets. Same for the heater, be sure the outlet is adequate. You can suspend your batting from the ceiling if you buy by the roll. Set it in a corner and a pole with chains works fantastic. This way they are not in the way taking up space. Be sure is away from the garage door though. LOL. Hope these ideas help.