Looking for advice on 2014/2015 Prius

Hi all, I’m not a Prius owner, yet... but will be one vicariously through my daughter who graduated from college in MI and will be moving to CA for graduate school. Time to retire her 2006 Pontiac Vibe (for which I’m an active GenVibe forum frequenter due to the 5 vibes I’ve owned over the years). With 5 drivers in the family, I’m switching to Prius, and am the kind of person that knows enough to be dangerous, but appreciate all the info at forum sites like this, so thanks in advanced. Here are my questions:

1. Giving our budget, under $15K, I think 2014/15 would give us the best deal for a Prius in the later stages of its generation. In general, in the Prius world, how many miles should we consider for a car in the 6-7 year range. I know that less is better, but at what point in mileage do you just say no to a 14/15, especially considering battery life. Any other years/mileage I should be considering in that price range? For example, a 2010 with really low mileage vs a 2015 with higher mileage?
2. Regarding battery, my biggest blind spot, how many miles on average before they really need to be changed? 100K, 125K... what happens at the 200K stage? Any other considerations regarding the battery? Is there a way to check its ‘health’ like you can with an apple iPhone?
3. Private party vs dealer? The type of loan we have means we most likely will go thru dealership, unless there is a compelling reason not to. If its a crap shoot either way, I’ll probably stick with a dealer.
4. What kind of maintenance can I expect to do myself? Are brakes basically simple disc type brakes or something special? Is it easy to find DIY fixers videos on YouTube?
5. Finally, what questions/issues should I be asking that I’m not.

Kind regards,
Paul