A reimagined cast iron building in the heart of  New York City’s Tribeca  has already become one of Manhattan’s most sought-after residences, with buyers snapping up almost every unit inside the property before construction had even been completed. 32 Walker Street is a seven-story, five-unit property that blends historic architecture with modern luxury, courtesy of an extensive overhaul that, when completed, will see a once-crumbling street corner transformed into a hot spot of design excellence. But while construction on the luxury residence—which combines an existing L-shaped structure with a newly built property—is ongoing, Serhant listing agent and “Owning Manhattan” star Peter Zaitzeff, who is representing all of the units, notes that interest in the finished result has already reached fever pitch.…