When it is necessary to build or repair a toll bridge on any county-district road, or to purchase any existing toll bridge on such road within any county, or across the boundary thereof, the county court of the county may contract for the same, or any part thereof, on such terms as may be agreed upon, and take bond and security from the contractors in the sum of fifty percent of the amount of such contract, and pay for the same out of the county road fund or out of the road funds of the district or districts in which such improvement is located, or by issuing bonds for the amount that may be agreed upon; and to this end the court shall appoint a competent engineer to make a thorough investigation and estimate of the cost of such improvement. Upon receiving the estimate of cost, the court, if satisfied with the correctness of same and the necessity and importance of such improvement, may advertise for and receive proposals and make such other orders in the premises from time to time as shall be necessary and proper.