The Prince William Board of County Supervisors gave final approval yesterday to the $1.4 million purchase of Rippon Lodge, a former Colonial-era plantation in Woodbridge that will be converted to a museum. The 17 room estate, set high on a 40 acre bluff near Route 1, is believed to be the oldest home still standing on the Potomac River. It has remained in the family of its builder Richard Blackburn, almost continuously since 1725. Owner Debra Black Decko, a District resident who grew up at Rippon Lodge, put the estate on the market last fall after maintenance and upkeep costs became burdensome. After Decko declined the county's initial offer, Prince William officials decided to use funds froma hotel sales tax to bid higher. Dozens of 18th century antiques will remain in the house, which needs some structural work. The county has not worked out details of the museum plan. This article was in today's Washington Post. June