Sounds like you found the perfect car for you. Even with a 7' frame, you're fitting in nicely. That's great to hear. BMW did a good job with the design to provide a lot of space, more than you'd think. For best value, you're looking in the right place. 2015's are the way to go. If you care about the packages, look for something that has 2 of the packages and less than 20k miles for 20-21k USD.
You can check my posts for my other detailed opinions that may help, but to respond to you questions directly from this thread:
Yes, the 2017 is the first year that the 94ah battery battery was released. The 2016 and older all have the smaller capacity battery. Yes, I'd recommend the 94ah REX for your situation. If all things are perfect, you may get away with a 2017 BEV to get your 85 miles since it's rated at I believe 114mi or so of electric range only. You'll have a bit left over to cover errands and higher speeds. If you're anywhere close to the max range though which you are, I'd recommend the REX. The BEV really is only for people who are driving within the City.
I'm assuming you can charge at either end of your 85 mile destination and will have the time to do so. You may want some range in between to run errands or what not. I can't speak to your situation specifically b/c your range will vary based on the terrain, hills, speed. I'll tell you my situation and you can judge your situation from there. I have a 2016 REX.
I used to drive 90 mi per day. Most of it was freeway. I'd say about 90%. Definitely inclines, but I wouldn't say steep hills of any sort. I can get to my destination 45mi away with a bit more than half the battery left. However, that doesn't mean I can make it back on what's left. Note that when you're at home overnight and it's on the charger, you can set it to pre-warm the battery which will provide you maximum range efficiency. That process takes 3 hours. I don't have that luxury at work after 45 miles to pre-warm the batteries before heading back home, so the efficiency returning home is less than the efficiency driving to work and that's what explains why I can't get back home even with more than 50% battery left. I'm sort maybe 12-15 miles or range and would need to use the gas tank to generate the additional electricity needed for the full commute back home.
No the car won't accommodate other sized tires without custom work. Some i3's come with a staggered setup, meaning the rear tires are wider than the fronts. You can't put the rear tires on the front or it'll rub.
A fanatic of all things electric. Ridables like electric ice chests and stand up scooters I adore. Drones like DJI, I root for. I'm a fan of both, but nothing beats my love for my 2016 i3 Rex with Deka World. Shhh, don't tell my wife.