I would surgest consider front wheel drive, I have found it has a few advantages, first you get 2 wheel drive, which is good going offroad, second the setup is more simple to mount and exchange tire and tube, third in my opinion, the balance of the bike is better than rear motor. I have had a rear wheel drive mtb, and the weight on the rear put high load on spokes and tire which lead to many broken spokes and much faster worn rear tires. On my latest FatBike setup with Smart Pie 5 in front, never had broken spoke and tire wear is very equal front/rear, and so far it has run more than 2800 km this year, flawlessly.
It runs 48v and with the fatty tires it runs 45-50 km/t without pedaling.