The county superintendent shall make an initial or replacement purchase of a motor vehicle, trailer, or other item of automotive equipment involving an expenditure of more than three thousand five hundred dollars ($3,500) through the county purchasing agent or he shall make the purchase directly from the vendor only after having secured in writing from responsible vendors at least three estimates of the cost thereof. The county superintendent of schools shall purchase the equipment at the estimated cost from the vendor who submits the lowest estimate meeting the prescribed specifications, or shall reject all estimates.