If the whole battery needed to be replaced, it would likely be a lot more than that figure! Other than infant mortality failures (not many), the battery does lose capacity as it sees more charge/discharge cycles, but, the factory warranty is to retain at least 70% for 8-years. Since the beginning, there haven't been that many reports of people needing them to be replaced, but official numbers are not available to back that up.
There have been some service campaigns that tweaked or replaced a few things over the years, but not really all that many...those are no-charge changes. You should be able to take the VIN and have a dealer tell you if any that applied to that vehicle were all done, and if not, you should get them done.
I find the car fun to drive and does hold up to four adults fairly comfortably. Especially for running around town (which is why I bought mine), it's good since I can tell it to warm/cool the interior from my phone or computer, and it's not likely to see the normal wear items on an ICE that only goes maybe 3-5 miles on a trip (oil contamination, short muffler life, etc.).
The turning circle is great as is slinking into parking spaces. At least in normal driving, you'll never embarrass yourself - it doesn't weigh much, has full torque from a stop, and doesn't lose anything since it has a single speed gearbox, so that's simple, too.
The BEV has less to go wrong than the REx and is speedier since it's not carrying around that extra 350# or from the engine, gas tank, oil, coolant, muffler, exhaust, etc., or the necessary maintenance any ICE has.
It all depends on what your expectations are.
2011 535i x-drive GT, 2014 i3 BEV