After a district shall have been established, the administrative head or body shall cause to be prepared by the county engineer, or other county officer having equivalent qualifications, or a duly licensed engineer employed for that purpose, detailed plans and specifications for the improvement, a careful estimate of the expense, and, with the assistance of the county attorney, or an attorney employed for that purpose, a proposed contract or contracts for the execution of the work. The administrative head or body shall examine such detailed plans, specifications, estimates and contracts and may adopt, modify, amend or reject the same. Upon adoption of the plans, specifications, estimates and proposed contract, the administrative head or body shall cause contracts to be let in the same manner provided for other county construction projects. Nothing herein shall prevent the purchase or condemnation of any existing county system, or portion or portions thereof, whether inside or outside of the county, necessary for the purposes of the county district, provided, however, that there shall be no power to condemn property the legal title to which is vested in a public corporation or a special improvement district unless the owner shall consent thereto. The cost thereof, together with the cost of construction of those facilities proposed to be constructed, shall not exceed the maximum cost of the project as advertised in the notice of hearing published pursuant to § 280-E (Public hearing)section two hundred eighty-e of this article. In the event that a system owned by a municipal corporation or existing district is purchased, the county may by agreement with the seller, assume the payment of annual installments of principal of, and interest on, obligations issued by the selling municipality to finance the cost of the facilities so sold. If payment of annual installments of debt service is not assumed, as aforesaid, the selling municipality shall set aside in a reserve fund, so much of the purchase price received as is sufficient to meet all future installments of principal of, and interest on, outstanding obligations issued by it to finance the cost of the facilities sold. Moneys in such a reserve fund may be invested as provided in General Municipal Law § 11 (Temporary investments)section eleven of the general municipal law.