The Secretary of the Interior is authorized to accept donations of land, interests in land, buildings, structures, and other property within the boundaries of said national battlefield park as determined and fixed hereunder, the title and evidence of title to lands purchased to be satisfactory to the Secretary of the Interior: Provided, That under such funds available therefor he may acquire on behalf of the United States by purchase when purchasable at prices deemed by him reasonable, otherwise by condemnation under the provisions of section 3113 of title 40, such tracts of land within the said national battlefield park as may be necessary for the completion thereof.