I don't like to use steel wool, the fibres can easily get stuck, and it is unpleasant to use, very dusty. Much better is a stiff brush. I have some with fine stainless steel bristles which are sold for stripping paint of old furniture. The kind with brass or nylon bristles that are used to clean suede shoes also work quite well. If there is corrosion on the sides of the reed slot that can be draped off with a scalpel blade, but I would only do that if the reed was not moving freely.
Plastic valves are good because they are always the same, and last longer. But they don't sound exactly the same. The sound can be a little brighter and for the highest reeds is may be good, but for the lower treble reeds and the bass leather is much the best. What I would suggest avoiding if possible is the artificial leather, sometimes called vileda, the material is a poor substitute for leather in my opinion.