I would go with either a medium or long cage Campy derailleur. Fortunately, fairly recent models (rated as 8 or 9-speeds) are commonly sold for greatly reduced prices. Now that 10-speeds have become increasingly more common and of course most popular with newer bike buyers, older models are just dead stock on bike shop shelves. The earlier models will still have the same generous reach in case you ever decide to update the frame to 130mm spacing for 8-9-10 speed cassette rear hubs. Just look for a lower tier model which will still have aluminum construction rather than carbon fiber paralellogram plates and it will maintain the more classic component look I assume you'd prefer. Keep them well maintained and derailleurs used on road bikes can last a lifetime.