I don't know a whole lot about 27's but fell comfortable with:
An outboard conversion reduces the value of the boat because it is not original and is not as reliable as a diesel. Unless the outboard is electric start and remote controlled, it is a pain to deal with on a transom such as the 27's. In rougher water, outboards will usually come out of the water and be slammed back into the water providing jerky forward thrust and stress on the lower unit. They are however lighter overall and less expensive to maintain. Also, the transom should have been beefed up since it was not designed to carry an outboard (were some 27's standard with outboard?)
Other than that; the usual rigging, deck, hardware, sails, bottom, hull, wiring and interior bulkhead considerations apply.