within the unit, if compatible with the purposes for which the unit is established, expanded, or designated by this Act, and the other provisions of this Act, or

outside the boundaries of, and in the vicinity of, the unit.

the Secretary and the head of the Federal agency having primary authority over the administration of any Federal land which the Secretary determines is suitable for use in carrying out such purpose may enter into agreements permitting the Secretary to use such land for such purposes;

notwithstanding any other provision of law, the Secretary, under such terms and conditions as he determines are reasonable, may lease or acquire by purchase, donation, exchange, or any other method (except condemnation) real property (other than Federal land), office space, housing, and other necessary facilities which the Secretary determines to be suitable for carrying out such purposes; and

the Secretary may construct, operate, and maintain such permanent and temporary buildings and facilities as he deems appropriate on land which is within, or in the vicinity of, any conservation system unit and with respect to which the Secretary has acquired authority under this subsection to use the property for the purpose of establishing an administrative site or visitor facility under subsection (a), except that the Secretary may not begin construction of buildings and facilities on land not owned by the United States until the owner of such land has entered into an agreement with the Secretary, the terms of which assure the continued use of such buildings and facilities in furtherance of the purposes of this Act.