Any unit of local government is authorized to voluntarily participate in this program. Any unit of local government which is authorized to issue, sell and deliver its local government securities under any provision of the Constitution or laws of the State may issue, sell and deliver such local government securities to the Authority under this Article; provided that and notwithstanding any other provision of law to the contrary, any such unit of local government may issue and sell any such local government security at any interest rate or rates, which rate or rates may be established by an index or formula which may be implemented by persons appointed or retained therefor, payable at such time or times, and at such price or prices to which the unit of local government and the Authority may agree. Any unit of local government may pay any amount charged by the Authority pursuant to this Article. Any unit of local government participating in this program may pay out of the proceeds of its local government securities or out of any other moneys or funds available to it for such purposes any costs, fees, interest deemed necessary, premium or reserves incurred or required for financing or refinancing this program, including without limitation any fees charged by the Authority pursuant to this Article and its share, as determined by the Authority, of any costs, fees, interest deemed necessary, premium or reserves incurred or required pursuant to Section 820-20 of this Act. All local government securities purchased or otherwise acquired by the Authority pursuant to this Act shall upon delivery to the Authority be accompanied by an approving opinion of bond counsel as to the validity of such securities. The Authority shall have discretion to purchase or otherwise acquire those local government securities, as it shall deem to be in the best interest of its financing program for all units of local government taken as a whole.