Before you get too deep into this, be certain you check with your city or county to make certain they will even allow it. If they allow it, they are going to have quite bit to say on how it looks and how it is built and out of what materials.
Also, the cost for permits, not simply building construction, but grading, road/driveway, drainage, building height all come into play. You mentioned a gravel drive. Some cities won't allow that. Also, consider the possibility that fire sprinklers may be required. Here in California you must have fire sprinklers on all new construction (residential).
Your first move should be to contact the city, not a contractor.