The best thing to do to the metal is hose it down with CLP and wipe it down. Use the CLP to clean off the metal and remove any rust. Do not refinish an old or historic firearm if you are wanting to maintain it's value.
As for the wood, clean it off with a good wood cleaner, but again be careful not to ruin the finish. With any wood cleaner, do a very small area that is hard to see to make sure your cleaner doesn't harm the wood.
What ever you do, do not try to reblue or refinish any metal work as this would really hurt the value. Same for the wood. The original patina is more important than making it look new. An old worn look that is original is more important than making an old historic weapon look new.
Breakfree CLP, Cleaner Lubricant, protectant. It's available in several grades. I use "collectors" CLP for long-term storage, and the regular stuff for every day cleaning and lubricating.
Brass wool only to gently remove rust on metal and oil only. Do not use stripper on wood, better off cleaning with denatured alcohol. You can use olive oil to clean any part as it will not react with anything.
I think it depends on just what it is and your intended use for it.
I would have no reservation doing cosmetic work on generic guns like a Ruger 10/22 or H&R SS shotgun if it is good mechanical condition. If you do it right you might turn a profit on it.
Even in these models if the gun was special in some way, maybe your grandfather's gun, then you might not want to alter it.