Don't worry about the rust on the outside. Worry about any rust on the inside. If water can get into them through an open vent hole or speedo hole it would have almost no chance of making it back out. That trapped moisture will cause damage (pitting mainly) to any finely machined surfaces. You should ask to remove the covers and take a look inside before buying. If there's rust on just the diff gears and you don't plan to use that ratio it's no big deal.