bought a car sight-unseen before. Actually, I've bought 4 that way. The second one was a major mistake but I learned a lot from that ordeal and while it didn't turn me off of the concept entirely, it did drive home how much more risk you're taking on as well as highlighted that buying 'locally' is way easier.
Assuming the link you posted is the actual vehicle in question, a few things jump out at me:
+ Good amount of pictures from most angles you'd want to see
- No pics of the dash, displaying the GOM and % (to roughly estimate battery capacity)
- No pics of mechanical related things: tire tread, lights working, any rust (oh, wait!
+ Car looks to be in good shape from pictures
- Freshly cleaned interior could be suspicious. Will it have a faint smell of something disgusting happening in there?
The biggest downside though, is the description says NOTHING about the condition of that vehicle. They describe the options it came with from the factory, then they describe their dealership. In my experience this is a major red flag. It also happens to be a major way of doing business because dealers don't have time to write up detailed conditions for every vehicle that crosses through the lot. For buying sight-unseen though, a detailed description of the condition of the actual vehicle as-is should be a requirement (and not just BMW's description of when that vehicle left the factory). You could talk to them on the phone, and Joe might be a really nice guy (after all, he has a 5 star rating!) and I know used-car salesmen are notoriously trustworthy, but bear with me here and remember: all legal protections are buyer-beware, and anything not written down doesn't count.
So, while I wouldn't absolutely write off buying a car online, I'd encourage taking a highly skeptical approach to any vehicle you're looking at like that. As eNate mentioned, you need to be willing and able to walk away from an online purchase if something feels wrong. Money back guarantee from Carvana or the like is a good way to do it. Maybe this dealer offers something similar, but even if they do, keep in mind such a guarantee is only as good as the company that issues it.
In this particular case, and with the information publicly available, I'd pass. Combine that with your uncertainty of whether you actually want those features or something else and I'd suggest waiting as well.
In regards to the REX vs. 2017; I'd suggest going for the 94ah battery unless you know you want the REX. I love the solution BMW came up with using the REX, but pure BEV is just so much simpler so if you can get away with it (and most people can), you're probably better off with BEV only.