ITA with the earlier suggestion to put a wall up to contain your room.
Also, consider the exhaust from the car ... and the moisture from melted snow which would also mean salt/calcium. NOt exactly a good mix with fabric, sewing machines etc.
There are some good heater units you can get that would work perfectly for your garage. Your room could be insulated easy enough, and without a lot of work ... and become more like a regular room in your home. A little extra work done, will make it more enjoyable ... and you'll be more likely to use it.
BTW ... those higher counters you mentioned ... perhaps they'd be the perfect height for cutting fabric? or an ironing centre? If they are too high for chairs, what about getting an adjustable drafting chair? Their lowest level is usually the highest level of a regular office chair. If that works, it might be able to use them your sewing machines.
ENJOY making your plans for YOUR space!
