At or before the time any such bridge shall be acquired by purchase the state road commissioner, he shall determine what repairs, replacements, additions or betterments will be necessary to place the bridge in safe and efficient condition for the use of the public, and to cause an estimate of the cost of such improvement to be made and submitted to the commissioner by an engineer or engineers appointed by the commissioner. Such improvements shall be ordered by the commissioner before the sale of any bridge revenue bonds hereinafter authorized for the purchase of the bridge, and shall be paid for out of the proceeds of such bonds.