It's not necessarily the number of miles, but how well it was taken care of and the abuse it was subjected to. Your eyes are the best way to tell if they need replacement. Are some of the teeth worn and others are not? Are the valleys between the teeth elongated? If you're going to get an o-ring, spend the extra $35 to replace both sprockets. And your new chain will come with a new master link.
I bought the "DID 428 Vector Supreme Drive Chain" from Dennis Kirk...http://www.denniskirk.com/jsp/produc...=11977120&mmy=