Potential doesnt need to be outstanding other than for planesman, which are exclusively used on SS.
For a BO, I'd settle for a 14 pot/11 rep/11 res.
As far as RN goes, get gunners that have a good reload stats, ideally 10/12, 11/12 if you can, or elite reload (11/13).
Reason is after the heavy gunner classing, they get a huge nerf to reload (since you normally class as Acc gunners then), so whatever little ability you can get, you want it.
For the rest, I,d suggest going to the nation specific subforum about the crew suggestion. People there will be able to explain to you the various possible setup there is, and their pros and cons.