After the said peace memorial has been established as provided in section 433a of this title the Secretary of the Interior is authorized to accept donations of land, interests in land, buildings, structures, and other property as may be donated for the extension and improvement of the said peace memorial, and donations of funds for the purchase and maintenance thereof, the title and evidence of title to lands acquired to be satisfactory to the Secretary of the Interior: Provided, That he may acquire on behalf of the United States out of any donated funds by purchase when purchasable at prices deemed by him reasonable, otherwise by condemnation under the provisions of section 3113 of title 40