Good advise here. When I bought my first double, a William Evans 470, my buddy bought a new Chapuis 470. I've had the gun for 6 years and would make a nice profit if I decided to sell. My buddy would be out about 1/3 of his purchase price. I've since bought 13 other doubles and sold 5 of them. All of them but one were vintage british doubles and I made a nice profit on 4 of the 5, the 5th one I sold for what I paid for it. I suspect if you sink more into a new gun than the base model you will tend to lose more.

The key on a vintage gun is to have it checked out throughly by a qualified gunmaker, not the local gunsmith before you buy.