The loans authorized by sections 164.121 to 164.141 shall not be contracted for a longer period than twenty years, and the entire amount of the loans shall at no time exceed, including the present indebtedness of the district, the maximum aggregate percentage, as set forth in Article VI, § 26(b) of the Missouri Constitution, of the value of taxable tangible property therein as shown by the last completed assessment for state and county purposes. The rate of interest upon the bonds shall, in no case, exceed the highest legal rate allowed by contract. Before or at the time of issuing the bonds, the board of directors shall provide for the collection of an annual tax sufficient to pay the interest and principal of the bonds as they fall due, and to retire them within twenty years from date contracted.