The worst part of what I am reading here is that you are in a rush to get a building up. That may cause issues with your expectations of when you want to start, the total price, getting materials & an unrealistic schedule to finish it. All it takes is one setback such as a concrete contractor who can't start on the date agreed to start a domino effect that could extend the project time by weeks or months. Is it possible to rent another space while you plan better for this garage build?