$13k is a super deal, even for one with a few extra miles. Not a lot goes wrong with a Prius, they are one of the cheapest to maintain in the category of non-scheduled maintenance. If there was any sort of problem with the battery the car would display a warning light, so it's probably just fine. There is a "health" meter in batteries, and that's the internal resistance and it can be calculated by monitoring the voltage of the battery as it charges and discharges, but this is not particularly easy to do, so don't expect any service people to be prepared to measure such a thing.
See
http://home.earthlink.net/~graham1/P.../BatIntRes.htm for some more info.