I always thought black smoke was indicative of an injector issue, ie too much fuel being dumped into the cylinders, not a "lower end" issue that a long block change out would address.
Since you are buying a boat for world cruising, you should definitely have the issue resolved before completing the purchase. You don't want to deal with this in Fiji! New engine for a 54 runs what about $20-30K? If the owner will come off the price go ahead and get a new engine. You may even be able to sell the old one to reduce the expense.
Regarding the "mechanics," they should be able to give you a definitive explanation for the issue before suggesting a new engine. That smells fishy to me. If the boat is reliable in the short term, ie can get you back to Lauderdale then maybe it would be good to get her someplace with better "mechanics" or even see if Joel will take a peek at it. I'm sure Joel can recommend some high quality honest mechanics to look at her.
Good luck, keep on plugging away at it!
Vent de Soleil SM37
Great Bay, Sint Martin