Legendary businessman Sam Zell’s Equity International acquired two minority interests in senior housing developers in Great Britain and New Zeland.   The two privately held companies that were invested in were London-based Renaissance LifeCare PLC and Sanctuary Residences of Auckland, New Zealand.   With aging populations and a lack of high-quality housing, retirement real estate presents a “considerable opportunity,” Equity International said in a news release. The two firms have properties in London and Southern England; Sydney and Melbourne, Australia, and Auckland, with plans to expand into new markets.