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You should be able to find a boat in pretty good shape inthe 27' range with your budget. Like this one:1986 ERICSON w/Oceanside Harbor Slip Sloop Sail Boat For Sale - www.yachtworld.com
As far as buying, upgrading and selling for a profit, it can be done but it is rare even for people in the business to pull off. If you buy a 25 year old boat everything you add to it will be at today's prices. The best investment is in pleasure, customizing a boat for yourself, to get just what you want. Especially in today's market with so many good values out there.