From my experience, you'll be able to use Facetime with any external USB camera, but no matter the resolution you won't be able to send Facetime video in HD.
The Logitech C910 seems to do 720p in Facetime on my late 2012 Mac Mini, but the Logitech QC900 Pro, which was the most reliable web cam I ever used in Windows, doesn't even work consistently on the same MM.