The Secretary shall require that the rents for the dwellings in any project insured under this subchapter shall be established in accordance with a rent schedule approved by the Secretary, and that the investor shall not charge or collect rents for any dwellings in the project in excess of the appropriate rents therefor as shown in the latest rent schedule approved pursuant to this section. Prior to approving the initial or any subsequent rent schedule pursuant to this section, the Secretary shall find that such schedule affords reasonable assurance that the rents to be established thereunder are (1) not lower than necessary, together with all other income to be derived from or in connection with the project, to produce reasonably stable revenues sufficient to provide for the payment of the operating expenses, the minimum annual amortization charge, and the minimum annual return; and (2) not higher than necessary to meet the need for dwellings for families of moderate income.