If I'm creating a server (nw6sp5) to move an existing GW7 post office
onto, what replica (edir 8.7.1) information do I need to have on it?

It's going to replace a server that has replicas from our whole tree.
The new server is going to have the same GW users on it as the old,
some of which are outside the replica 'scope' of the one replica I was
going to put on this server.