Your situation is the same as mine was just a couple months ago, (discovering E bikes plus this site was like turning over a rock and seeing ants scurrying around, the innovation and activity in play blew my mind) and in short order I transitioned from my first buy (a ready to ride 36 volt hub motor), to a BBSO2 mid drive conversion to a mountain bike I already owned, to my third (and last, at least for a while???) build. A fat tired brand new bike with the BBSHD drive. I am also in the middle of hot rodding up the 36 volt hub motor first ride just because it sucks so bad as compared to my other two rides. All three then will have 52 volt batteries, interchangeable. If I had it to do over again....I would have passed on the first buy, but I didn't know what I was doing, and really it's OK it's just that the other two are so much more then OK. The so far brief (still a lot of snow here) riding with the fat tired BBSHD shows it to be capable of almost vertical climbing, I haven't tried any stairs yet but feel they won't be a problem! Less then 30 mph top speed due to it's low gearing, my "town ride" with the BBSO2 is geared higher, and easily gets over 30 plus is lighter and feels more bike like over all.
I have not taken the fattie into town yet, no reason to, but I think I may not like it riding around on pavement, as compared to my smaller tired MB, I mean why drag all that extra rubber around, just to look cool? You may ( I assume the funds are unlimited :? ) want to aspire to a full blown low geared trail rider, AND a higher geared small tired much faster town bike, anything less involves a lot of compromises, make sense? And don't even consider anything less then the increasing common (for me anyway) 52 volt packs. Two bikes and two batteries, so you can take both on longer rides, but on shorter rides not be packing the extra weight of a monster single pack, that's my plan anyway.