One of the last residences at the Stanhope conversion, the former hotel where the late John F. Kennedy Jr. reportedly stayed during rocky times in his marriage, is now in contract for $23.5 million. We hear the buyer is a New York-based businessman. The 8,300-square-foot, full-floor flat, with five bedrooms and 7 1/2-baths, is on the 15th floor of 995 Fifth Ave. It has fabulous views of the Metropolitan Museum of Art and Central Park. Another unit, on the 16th floor, is asking $28.5 million.