Best advice, get the highest grade concrete possible. standard residential/garage grade doesn't usually meet the tensile strength recommended for two post lifts. and go thick, 8 inch minimum. FBi buildings is a decent design your own building company.
Design Your Own Pole Barn | FBi Buildings
Kentucky Steel Buildings is another one, they sell custom metal pole barns but instead of wood trusses etc, they use Steel trusses for not much more.