Upon the completion of said survey and investigation, the district board shall cause to be filed in its office a written report of the same. Said report shall specify the character of the proposed improvement to be made, or property or rights to be acquired, shall state in reasonable detail the probable cost of same, including integral parts thereof: PROVIDED, That such estimate of the cost shall be held to be preliminary only and shall not be binding as a limit on the amount that may be expended in carrying out the proposed project. Said report shall also outline a plan for financing the proposed project, shall contain any recommendations that may be deemed advisable, and shall be identified by the signature of the secretary of the district as the official report of the survey and investigation in the proceedings to organize said improvement district.