Stug's weren't designated tank destroyers, but they were often used as them. That was the whole purpose of upgrading to the long barreled 75mm cannon: to increase anti-tank capabilities. Otherwise, Germany would have left them armed with the L24.
The M10 and M18 both used essentially the same gun as the M4A3(76)W, and so all you would get by using one over the M4 is speed, and not a great amount with the M10.
Here is a revised list:
Perked
Jagdpanzer L70 (50 base)
Jagdpanther (70 base)
Panther (60 base)
Elephant (100 base)
Free
Nashorn
Jagdpanther L48
Hetzer
Stug III ausf G.
M18 Hellcat.
The Nashorn is listed as free because of its crappy armor, small ammunition reserve, and because its gun has a limited traverse.
I'm dead serious about the Elephant too. Of course we would have to get a Nashorn at the very least to kill it(its KwK43 L'71 is about the only thing that would be capapble of killing one from the front, given the 200mm armor). Of course, the Tiger I, Firefly, Panther, and Jagdpanzer L70 would all be capable of killing one from the side, but you can't have a tank that is invincible from the front.