I'm not sure about this, but I am doubt AMEX will offer you the Centurion just because of one huge purchase. They might look at your spending history and see that you will not normally meet their spending requirements. If you don't have to pay the 3.5% (at least that is what my company pays to accept AMEX) then there is no risk in your plan. But spending an extra $12,000 is a pretty big risk.
I have never done this, but do car dealers generally accept credit card payments for the whole car? I once paid a car payment with a card and it went as a cash advance with lots of fees and no miles.