The board of each forest preserve district has the power by ordinance to purchase or lease as lessee real or personal property, or to acquire an easement in real property, for public purposes pursuant to contracts or leases which provide that the consideration for such purchase, lease or easement may be paid in annual installments; and to lease as lessee or to purchase real property or personal property for public purposes pursuant to a lease or purchase agreement which lease or purchase agreement may provide that the district may, at its option, purchase the property which is subject to the agreement or lease upon terms wherein payments previously made, or a portion of them, are deducted from the purchase price of the property as provided for in such lease or agreement.
     However, the maximum amounts that will become due under the terms of such purchase or lease agreements, or under the terms of the acquisition of an easement, together with all other indebtedness of the district, must be within the constitutional limitations upon the incurring of indebtedness for such district and for such pertinent public purposes.