The Libra Foundation has since October spent $750,000 buying a dozen houses, a community center and a general store along Monson’s main drag — as well as a farm on North Guilford Road, said Erik K. Hayward, Libra’s senior vice president. The plan is to convert the houses into artist residences and the center into studio space. The store would sell art and produce from farms in Piscataquis County, which the U.S. Census Bureau rated as Maine’s poorest in 2015.