§ 121. Acquisition after project approved. When a project has been approved, an authority may purchase, lease or condemn, or obligate itself to purchase, lease or condemn, real property for a project. Notwithstanding any other provisions of and in addition to any other powers granted under this chapter, where all the dwelling units in a building have been leased by an authority for occupancy by persons or families of low income in accordance with the provisions of section twenty-three of the United States housing act of nineteen hundred thirty-seven, as amended, the prior approval by the local legislative body of such leasing program shall constitute all necessary approvals under this chapter for an authority to exercise an option to purchase such real property.