Victor Sigoura’s Legion Investment Group is planning a 20-story, 75-unit luxury condominium at 550 West 21st Street in West Chelsea. The site, purchased for $87 million earlier this year with a $56 million loan from Deutsche Bank, is considered the last undeveloped waterfront parcel in the neighborhood. Legion may add 11,000 square feet by contributing $7.8 million to the West Chelsea Affordable Housing Fund.