I've finished both main campaigns (Privateer and Righteous Fire) plus the side campaign (Syrai and Soren branch), and have still yet to see a Drayman, despite actively looking for them every time I'm somewhere with a ship dealer. I especially want one so I can see what kind of money there is to be made buying and selling ships; I want to try flying a Paradigm soooo bad, but I also don't want to hex-edit my save to get the needed credits.
As for using a Demon...yeah, they're fun as hell to use, but overall, I think the Centurion is a better choice, since you have the same four guns, but also 4x the missile capacity and L3 shields instead of L2. (Plus, the Demon's reactor and shield capacity don't go up when you reach Righteous Fire; the Centurion's goes up to 4 and 4.)