IMHO I would go with a poured concrete foundation (normally used in my area of the north east) or the block foundation if that is used in your area. Concrete is going to be there a lot longer than wood even if it is pressure treated. The important thing of course is to make sure you go below the frost line and put the proper footings. Your local building officials will tell you what is needed. Then you can go with a traditional stick framed building. One thing you may want to keep in mind using the space above the garage as storage. I did this in my garage that I built last summer. I used I-Joist for the ceiling/second floor so that I would not have to have any posts in the middle of the garage. Good luck in your project.