Hi, I ride a 2016 FX so my opinion might count, or not. I would advise against the single pack unless your needs are very limited, as in short commute, little or no freeway. Range with one pack is very limited and the power is less at freeway speeds. But it is light and inexpensive.
Two packs is better. I like modular but it is now an extra cost so the 7.2 non-modular might be best, again depending on your needs. I wouldn't worry about upgrading the batteries, they only improve incrementally and 5 years is a great warranty.
Personally I went for the works and bought four packs and two external chargers. I ride off road mostly and to keep up with my friends gas bikes, I do 40 miles of trail in the morning, swap packs at lunch and ride the rest of the day, then charge all four overnight, two on the bike and two external. Then I'm ready to do it again the next day. That means I always have the range I want and it means I spent a lot of money.
I do ride with one pack sometimes. I have some single track on my own land and the lower weight is nice when range is not an issue. But for anything over 15 miles I use two. The power drops noticeably when the charge gets low.
It's a great bike, you'll love it, although the suspension doesn't match KTM or Alta. Hope this helps you to decide.