I generally use an alcohol (rubbing alcohol...isopropanol) wipe and cotton buds (Q-Tips) on the exterior to remove finger gunge and tobacco film. Glass cleaner (as noted above) is also a good option. Don't get sloppy with the cleaning fluid. You don't want it to go inside the camera. Scratches to the silver finish cannot be removed. A certain amount of "brassing" or scratching is expected on most cameras that have seen any amount of use. This is particularly true of the bottom plate if tripods are part of the camera's history.
It is possible that some of the paint from engraved lettering may dislodge during the cleaning process. If that happens, I have found that a lacquer pen or crayon works well for touch-up afterward. You can also get creative with the colors to enhance its value later on eBay
Regarding black paint...It can be done, but the silver finish must be completely removed down to bare metal. There are online articles that detail the process, though it is my understanding that it is difficult to get a finish that will be truly durable. All metal-body black cameras, even those with anodized finish (rare) will begin to "brass" on the first day of use.
Steve