Welcome Ali I am unsure about the black friday sales but I am sure you will love the D810. I am also unsure of the PL mount/adapters. Someone may be able to help you soon.
I can't see the D810 being in any sale pre Christmas, it's a bit too new imo. I think the price may dip a little afterwards, that's what i'm hoping anyway, I've already sold my D800 in anticipation of this happening