When the tax has been collected or the application filed as provided in ORS § 551.080, the county court shall advertise for bids on the proposed work, either as a whole or in parts, as may be deemed most economical. The advertisement for bids shall be published in the official county paper once each week for two successive weeks previous to letting the contracts. The contracts shall be drawn in such terms as will insure the district against loss, and a bond required of the contractor such as will insure the completion of the work in case of failure to comply with the agreement. No contract shall be awarded which calls for an expenditure of a sum of money greater than the tax levied for the purpose; provided, that the difference may be made up by personal subscriptions; and provided also, the court may modify the plan so as to bring the cost of work within the tax levy, if such modification can be made without material detriment to the improvements.