Investors have bought four mostly empty buildings at Coronado Canyons for $12.25 million. The Nigro family and brokerage IREPLV plans to turn their section of the Henderson retail complex into a medical office park. Their buildings went into foreclosure during the recession after Coronado Canyons’ construction lender was shut down. It was one of countless real estate projects around the valley that ran into steep problems after the economy crashed.