By Rory Carroll SAN FRANCISCO - The Golden State Warriors basketball team on Monday acquired a 12-acre site in San Francisco's Mission Bay neighborhood where it plans to build a new, 18,000-seat arena for the reigning NBA champs. The Warriors' plan is unique in that it would be the only completely privately financed arena or stadium in the United States over the past two decades. "The Warriors are making an unprecedented, $1 billion-plus investment in San Francisco," said Rick Welts, president and chief operating officer of the National Basketball Association team based in Oakland, California.