The total aggregate amount of said notes outstanding at any one time shall not exceed a limit of 2 percent of the assessed valuation of the taxable property in the district or, if the assessed valution is not obtainable, 2 percent of the county auditor’s estimate of the taxable property in the district evidenced by his certificate, provided:
(a) A district which has been formed for less than 18 months may exceed said limit and in the event may borrow an aggregate amount of not to exceed thirty-five thousand dollars ($35,000).
(b) No district shall borrow more than an aggregate amount of one hundred fifty thousand dollars ($150,000).