The legislative body shall insure that such method or plan of the agency for the relocation of families or single persons to be displaced by a project shall provide that no persons or families of low and moderate income shall be displaced unless and until there is a suitable housing unit available and ready for occupancy by such displaced person or family at rents comparable to those at the time of their displacement. Such housing units shall be suitable to the needs of such displaced persons or families and must be decent, safe, sanitary, and otherwise standard dwelling. The agency shall not displace such person or family until such housing units are available and ready for occupancy.