There is a great book for questions like these:
Megayacht Wisdom II by Buddy Haack & Doug Hoogs. It includes chapters on new builds, refits, etc., and is written for owners to help them avoid the common pitfalls. It is available from
Bluewater Books. (I have no connection with the book or it's authors.) I have found it useful, as a yacht captain and naval architect, and recommend it to new or existing owners alike.