Consistent with other requirements of national defense, any permanent housing constructed pursuant to the authority of this subchapter shall consist of one- to four-family dwelling structures (including row houses) so arranged that they may be offered for separate sale. All housing of permanent construction which is constructed or acquired under the authority of this subchapter shall be sold as expeditiously as possible and in the public interest taking into consideration the continuation of the need for such housing by persons engaged in national defense activities. All dwelling structures of permanent construction designed for occupancy by not more than four families (including row houses) shall be offered for sale, and preference in the purchase of any such dwelling structure shall be granted to occupants and to veterans over other prospective purchasers. As among veterans, preference in the purchase of any such dwelling structure shall be given to disabled veterans whose disability has been determined by the Secretary of Veterans Affairs to be service-connected. All dwelling structures of permanent construction in any housing project which are designed for occupancy by more than four families (and other structures in such project which are not sold separately) shall be sold as an entity. On such sales first preference shall be given for such period not less than ninety days nor more than six months from the date of the initial offering of such project as the Secretary of Housing and Urban Development may determine, to groups of veterans organized on a mutual ownership or cooperative basis (provided that any such group shall accept as a member of its organization, on the same terms, subject to the same conditions, and with the same privileges and responsibilities, required of, and extended to, other members of the group any tenant occupying a dwelling unit in such project, at any time during such period as the Secretary of Housing and Urban Development shall deem appropriate, starting on the date of the announcement by the Secretary of Housing and Urban Development of the availability of such project). The Secretary of Housing and Urban Development shall provide an equitable method of selecting the purchasers when preferred purchasers (or groups of preferred purchasers) in the same preference class or containing members in the same preference class compete with each other. Sales pursuant to this section shall be for cash or credit, upon such terms as the Secretary of Housing and Urban Development shall determine, and at the fair value of the property as determined by the Secretary: Provided, That full payment to the Government for the property sold shall be required within a period of not exceeding twenty-five years with interest on unpaid balances at not less than 4 per centum per annum.