At under $20K, almost any 30-35 footer is going to need some work. That said, there are lots of boats from the 70s and into the early 80s that will fit your criiteria. For cruising LIS to Block and the Vineyard, a 30 footer is fine. My wife and i did it for 10 years in a Pearson 26.
Since winds on LIS are light during the summer, a somewhat newer racer/cruiser will allow more time sailing and less time motoring. A boat that has had the engine replaced in the last 10 years plus newer sails and no major structural issues is what you should be looking for. Read SD's sticky on boat inspection tips on this forum before you out looking.