Replacing the Toyota Prius Batteries?

Hello. Just a quick question for any Prius owners out there.

Can you tell me roughly what the real life expectancy is for the batteries inside the Prius? In years and/or mileage?

I understand that the batteries are made up of numerous different cells?

Do you know how many cells they are made up of, and how do you know when they need replacing?

Can you just test and replace caput cells individually or is it best to do them all at once in one go? Or in group of a fixed number?

I was thinking if you can do them individually as they go caput then it would be less costly and your costs for replacment would be over time.

I don't actually have a Prius but am considering getting a used one as my next car or possibly an Auris or Insight hybrid. I have seen single cell batteries for sale on ebay listed as Prius batteries that were I think ?25 in price.
Thanks Jerry! I'm against Diesels on principal b/c they give out much more pollution on a local level than petrol could hope to. Stuff that is 100 times more noxious and nasty. They cause bronchitus, asthma other respiratory illnesses & cancer. I'd be a hypocrite if I bought a diesel & claim to support fight against cancer.