As for the finish I suggest that you look up METAL FINISHERS in your local trade or engineers directory and get the whole thing chemically blacked. It is a chemical etching/staining process that is far more durable than any paint.
To be honest I would not be the least bit concerned about the lack of matching numbers because these numbers only relate to the barrel, barrel locking nut and butt slide when it is a working service gun when the numbered parts must remain together for technical reasons. For your gun this feature is totally academic. Remove them all except for the master number on the top rear of the body.
I would not do anything to the butt and pistol grip except give them a coat of darker stain and soak them in linseed oilicon. It looks like they have been sanded to death already. You could LIGHTLY sand the carrying handle and oil that.

It will make a good conversation piece if it's done well and properly. But it is your Bren