Since you're having your car built can we assume that you'll be having it painted professionally? For me, satin black (or primer black) is a trend that is coming to the end of the popularity curve - it hit California, has progressed across the country, and has been seen so much it has become common and people are moving back to the shiny finish again. Others here may disagree, but for me it would be sad to pay big dollars for a pro paint job to look like primer, but everyone has their "sweet spot" on what looks good and that's what makes old cars cool. Someone like "Hotrodpaint" can give their professional opinion, but any good paint guy can add "flattener" to any color top coat to yield a satin finish, and it will stand up just as well as anything else.