I don't think I would trust my wine to an old barrel that was left to mold. But if you must, make sure it doesn't have ANY off aromas. I would make a strong solution of sulfite and citric acid and fill the barrel with that. Let it sit for a few days.
There will be other potential problems with a used barrel. Do you know what it was used for in the past? I presume it has no more oak to give up to the wine, so it will just be a neutral barrel. Are you looking for the microxygenation? Will you add oak cubes to the wine? And, if it has been stored empty, it will likely leak.
Sorry to burst your bubble but you would probably be better off using this as a planter and buying a new barrel.