The 2016 Prius is the second Prius I have owned. The first was a 2007. Using the Econ setting, I am getting 57 around town. Have not taken it one trip yet, so don't know hwy mileage. I have only had it for 2 months, so no problems yet. My 2007 never gave me a bit of trouble. Just drive it and you should never have to worry about the big traction battery. I have known several owners of older models that had over 250,000 miles on theirs and never had a bit of trouble with the traction battery. I changed the 12 volt battery after 6 yrs of usage simply because I wanted to. And I always used full synthetic oil in the engine. I did have the plugs changed at around 85,000 miles, but noticed no difference b4 and after.
The design is a bit different, but similar to many new cars now. The 2016 is much quieter, handles much better, more powerful, (if one can use that term with a Prius,) and gets about 8 mpg better than the 2007 that I enjoyed for many years. Some of the interiors I thought were quite different, but I ordered mine with black and I think it is quite traditional. The dash is very different, but can b configured to give a lot of info. Prius generally redesigns every 3-5 years.
There is very little available in the aftermarket.
Remember that certain things (like a spare tire) are only available on certain models. I fortunately discovered that b4 I almost bought a 2 Eco model. Good luck.