Before submitting any proposition for borrowing money for the purposes mentioned in section 17-150, the mayor and city council of a city of the second class shall determine upon a system of sewerage and shall procure from the city engineer an estimate of the actual cost of such system and of the cost of the portion of such sewer as the mayor and city council may propose to construct, with the amount proposed to be borrowed and the plans of such system. Such estimate shall be placed and remain in the hands of the city clerk, subject to public inspection during all the time such proposition to borrow money shall be pending. After such system shall have been adopted, no change shall be made to such system involving an expense of more than one thousand dollars, nor shall any other system be adopted in lieu of such system, unless authorized by a vote of the people.