When you say your Luger has plastic grips, are you talking about the black bakelite grips? If so don't change them out, leave it all original. A wooden stock would not be correct for this sort of Luger rig, but if you choose to get one I suppose you could use it as you choose.
I don't know a lot about the stocks, but if original they can be quite pricey. I would go for the snail drum
