If you’re going to buy on Gramercy Park, a south-facing, fifth-floor view is ideal: flooded with sunlight, pin-drop quiet, and still low enough to graze over the trees. On the fifth floor of 1 Lexington Avenue, a 28-foot-long living room has four windows with this perfect, park-facing view — the star feature of a duplex that has not come on the market for 50 years, says broker Nadia Bartolucci, who added that it’s been under the ownership of a single family.