Frame material: If cost is no object, Carbon from a reputable builder. If cost is a consideration, Aluminum.
Frame Builder: Several good ones already mentioned. You do not need a custom frame. We are almost exactly your heights and weights and could easily adapt a stock "small" Co-Motion to our accustomed fits.
Brakes: We have three tandems with caliper brakes and a triple with discs. For racing, use caliper brakes. They are lighter and simpler. On a custom frame you can specify a rear disc mount just in case your needs change.
Wheels: As a serious racer, you are probably used to having separate sets for training and racing. No reason why you shouldn't do the same with your tandem. If the cost seems prohibitive, spend less on your frame.
Stoker Bars: zonatandem's advice is sound. Start with the stoker's favorite drop 'bars and add dummy levers. They will feel most natural to your stoker.