Taking it apart is totally simple and self-expanatory. I wouild think, for what you have to check, the stock can stay on and all you need is a Phillips screwdreiver to take the forend off and remove the barrel. If you want to remove the stock, a 9/16 socket and a long extension are necessary. I think my extension is 18 inches, and works well.
If the serial number's last three digits match those engraved on the lug, I'd pack it up and send it home to be fixed. It's a shame you didn't spot it before you bought it. I wouldn't even clean it, I'd return it as is and gripe about how filthy it is.
Pete