Looking at a BMC GF-01 - sizing advice?

Hey all,

I have the opportunity to buy a well-priced, barely-used BMC Gran Fondo — been watching for an endurance bike spec'd like this for a while. I'm 5' 11", 165 lbs and have ridden on everything from a 54cm to a 57cm bike in the past.

This BMC is 51cm. I've been reading this afternoon...