ATHENS (Reuters) - Greece is set to purchase back about half of its debt owned by private investors, broadly succeeding in a bond buyback that is key to the country's international bailout, a Greek government official said on Saturday. Greek and foreign bondholders offered the targeted 30 billion euros ($38.8 billion) in the deal, which is central to efforts by Greece's euro zone and International Monetary Fund lenders to cut its debt to manageable levels. "The buyback went well in broad terms. ...