The authority shall let contracts for construction in the same manner, so far as practicable, as is provided by law for contracts of the city, including but not limited to section one hundred three of the general municipal law. Nothing in this section shall be construed to limit the power of the authority to do any construction directly by the officers, agents and employees of the authority. Contracts for the purchase of supplies, material and equipment shall be let in the same manner as is provided by law for contracts of the city.