Hi Gdread,
I still enjoy a petrol powered off road experience! I have not managed to persuade any of my motorcycle mates to give e-mountain biking a try yet but I will keep plugging away.
I would say the purpose built mountain bike offerings from the likes of KTM Bosch crank & Panasonic hub, Bosch/Yamaha Haibike and as a curve ball the rear hub motor Oxygen Mountain bike all look good. I bought my Haibike sDuro from a shop that was close to me and that has proved very useful when you need help.
Ride as many as you can before you make up your mind. Although crank bikes are very much in vogue at the moment and very good a rear hub motor can be more relaxing and kinder to any injured knees.
After 7 months and 1600 miles I have so far seen no noticeable degradation in my battery. I look after my battery by bringing it into the house after every ride and charging it if I am going to use the bike the next day. If I am not going to use the bike for a number of days it will have over 50% remaining after my ride home from work and I store it without topping it back up in a warm dry cupboard and then charge it up again on the day I next ride.
Researching bikes before you buy is good fun and can take over your life!