UnitedHealthGroup, Inc., the biggest U.S. insurer, and McKesson Corp., the largest drug distributor, are vying for billions of dollars in added sales by bulking up their information-technology units. The companies are expanding beyond their core business to help insurers cope with the health overhaul, which threatens to swamp industry computer systems, said Joanne Galimi, a Boston- based analyst for technology researcher Gartner Inc. Bracing for added taxes and regulations, insurers are upgrading long-neglected systems used to enroll members, track care and process claims, Galimi said. They also face a 2013 switch to a new government-mandated system for classifying diseases. Insurer spending on data technology will jump 24% by 2013, to $11.3 billion, according to a March report by IDC Health Insights of Framingham, MA.