I purchased my first motorcycle earlier this summer, and it was a 2018 Zero S ZF7.2. I didn't really know how I would use the bike, and now that I have a summer of using it as my primary vehicle, I can share some of my thoughts.
First, I have never used fast charging but I totally see the appeal. When I plan trips that are on the outside of my range, the ability to stop at a station in between and add 15 minutes of juice would make all the difference. Now a ride where I'm going 40 in a 55 and tucking uncomfortably to try to maximize range, is enjoyable again. Not to mention that the fast charging opens up the option of going further than my total range with planned stops. It really seems like it would be a game changer.
I was told I could add the Charge Tank after the fact, which is false. I would very much like the Charge Tank option, so I'm a little bummed I'll have to wait for my next bike to have it as a factory option. I am very likely going to do an offboard, DIY solution in the spring.
The other takeaway I have, is I really wish I'd gotten the ZF13. I didn't see why I would need to go more than 70 miles between charges, but I have gotten home at 0%, including being limited down to 30mph for the last mile, and I get it now. You can't have too much range, IF you think you will use the bike that way. For me, I am using the bike in 100% of situations where I can use the bike, and that is greatly influenced by its range. If you have a planned use for the bike like a 20 mile commute, and you will use a car or different bike for everything else, YMMV but I'd still highly consider more battery as it really opens up the usefulness of the bike.
Lastly, I would lean towards newer models if possible. There seems to have been a lot of growing pains and things get better all the time. The ability to do your own firmware updates is a big selling point on 2017+ for me.