The Secretary of the Interior is authorized to accept donations of land, interests in land, buildings, structures, and other property within the boundaries of the said historical park as determined and fixed under this subchapter, and donations of funds for the purchase and maintenance thereof: Provided, That he may acquire on behalf of the United States out of any donated funds, by purchase at prices deemed by him reasonable, or by condemnation under the provisions of section 3113 of title 40