I was initially thinking I would need the range extender, but now I'm not sure. A couple of rides in and after totaling almost 4 hours of ride time with mostly on Eco and a little on Trail settings, I still have almost 50% battery left. Unless you're doing truly epic rides with tons of climbing, you may not even need it. It's pretty pricey.
The M20 is a great build. I initially ordered that one too and then the shop here was getting none in for who knows how long, but were getting some M10s so I just bit the bullet and after approval of the family CFO, got the M10. You really do get a lot of upgrades with the M10 that are worth far more than the upgrade cost, but I'm certain that I would have been VERY happy with an M20.