In the US market, the value of a refinish on a model 12 will drastically change the value of the gun just because of the collector market and overall attitude towards Winchester. Doesn't seem to change much here in Canada. You say yours is a 3" magnum. You may want to check to see if there is any original value in a 1950's "Heavy Duck" model. Best to be informed before you start restoring it.
I have owned three model 12s. One was refinished before I got it and I sold it. It had been reblued by someone who polished it too aggressively, wore off the edges, and it turned a deep plumb colour. This is a trait of the Winchester steel if you don't hit the correct temps and use the right chemicals. I refinished a 1921 nickel steel model myself by hand polishing to preserve the stampings and edges then rust bluing it. I was pleased with the result. I also have one I had professionally blued that turned out a beautiful deep black and looks brand new. I am a fan of restoring them, but only if there is no value in them unrestored and it's done properly.