That the board of directors, subject to the limitations contained in this act, shall have the power and it shall be their duty to determine what public works and improvements are necessary and whether the cost thereof shall be paid by issuing bonds to be paid by a general levy or by the levy of special taxes or assessments, or by issuing revenue bonds as a lien on revenue producing works or improvements, but before any bonds shall be issued, special taxes levied or liability of any kind incurred by it, the board of directors shall cause surveys of work deemed necessary by it to be made, and plans and estimates made by a competent registered engineer, who shall make a written report thereof showing the amount, character and kind of work to be done and the location and probable cost thereof, and if the works are of revenue producing nature the engineer shall include in his report his findings as to operating and maintenance costs and number of customers required to make the works self-liquidating within a reasonable period, and file same, together with plans and specifications in the office of the secretary, which report shall at all times be open to inspection by taxpayers of the district.