Pretty sure you’ll be taxed on this vehicle with REx. Check out Government site to check prior to purchase. 2015 REx for example will be zero road tax.
Back to your dilemma. Charging infrastructure is changing all the time. The larger battery helps of course, but back up REx is attractive. The i3 is fun to drive, but the suspension is unforgiving. Watch ones with 20” wheels. Standard 19” wheels are fine.
If possible get all the toys. Harmon Karon sound system and leather interior. Some of the cloth interiors look great but are impractical in the UK in our inclement weather.
Back to REx. It needs annual service, so oil change, etc. Run it regardless, at least a couple of times a month. Warning messages such as engine management light can be expensive. Known faults are tank pressure sender, hopefully resolved on newer models and fuel pump relay that lives under passenger dash is inexpensive fix. There’s also a NOx sensor if it all goes south. Uses a good Indy not main BMW dealer unless you want to spend cash and replace parts until they find the problem. If your spanner handy, then BMW ISTA software is sound investment, clearing parking sensor faults to airbag light resets.
2015 i3 REx, 60ah, solar orange. Reversing camera, leather interior, sunroof, park assist, professional navigation.