The problem with rust, is that if you don't either get it all off or most of it and seal the rest, it'll just start growing again. The best way is to abrasive blast it and then seal it with some type of quality firearms finish. If you want to make it look cosmetically decent, you can fill the pitting with an epoxy and then do some "body work" on it. The epoxy would be invisible under a spray on finish.
Another method is to use electricity. You'll need some washing soda (Not baking soda), a plastic container and a DC power supply. Mix the soda with water in the container. Use an electrode for the positive side. You can use any conducting metal but it should have a large surface like a strip of iron, etc. The negative lead goes to the part you want to clean. The dirt, crud, rust, etc leaves the negative side and moves to the positive side. I have used this method on a really fouled and slightly rusted rifle bore with excellent results. Use caution. It's electricity and water, plus: It will remove metal.
Now having said all of that: How rusted is "very rusted"? As bad as this one?
http://www.shootiniron.com/rusty%20870.jpg
Or this one?
http://www.shootiniron.com/P1010042333.jpg
http://www.shootiniron.com/P101XX0002.jpg
Very often, the rust isn't as bad as it looks but if you let it go, it will get worse than it looks!
As far as the worth of the pistol.....well...uhmm...A year ago, it was worth a LOT less than it is now. A year from now, it may be worth more than it's weight in gold! Keep yer powder dry, Mac.
Tuff-Gun Finishes. The Name Says It All.
Mac's Shootin' Irons
http://www.shootiniron.com