Boston-based Beacon Communities has closed on the buyout of National Development Corp. The $250 million purchase includes 59 properties, most of which are affordable communities, covering 14 states. Built between the 1970s and 1980s, the acquisition adds 5,300 units to the firm’s books. Prior to this deal, Beacon owned 60 communities throughout New England, Pennsylvania, Maryland, Virginia and New York totaling 12,200 units. The merger doubles Beacon’s staff to more than 1,000 employees and enlarges its third-party property management under the NDC Real Estate Management brand consisting of more than 120 properties and 7,250 units.