BERKELEY — An area real estate investment firm has bought a Berkeley apartment complex in a deal that’s expected to lead to upgrades for the property. One-bedroom units average 706 square feet and two-bedroom, two-bath units average 1,108 square feet. Built in 2010, the apartment complex originally was a condominium hub. With the recent deal, Sack Capital Partners has widened its purchases of apartment complexes in the Bay Area. On Aug. 29, it paid $52 million for the 164-unit Fountain Park apartment complex in San Jose.