If it is custom built, then the builder and designer reputations are the ones to ask about. Nominally, only really rich guys can afford boats to be custom built for them, so their boats could be very good indeed, or they were when new.
However, watch out for boats from companies like Colvic. They made the hulls but others finished them, sometimes very well, sometimes not.
Then there are all the home made boats, especially in ferro-concrete. You need a really good surveyor to judge them for you.