I believe Singer is a small, highest-end car customizer who only really appeals to Porsche people. There are plenty of those in the world, but the Porsche lovers who also like watches are also sought after by Porsche themselves and a variety of other watch brands who do a "special Porsche edition" or homage. You may say 15 is a small number but the price is a big number and that rules out most who might be interested.
I say no, it won't be collectible because as a watch company Singer is insignificant. And the movement is used elsewhere. It takes too much explaining... just buy another Porsche instead and no explanations will be required. 
That's my 2¢.
Mike