I am 18 stone (again)and my Bosch KTM achieved 25 miles no problem. from what I read on German forum most average 50 - 55 km but many a lot more. Remember it is a fast charge battery 90% charge in 1.5 hrs and full in 2.5 hrs. I would start with one battery and see how you get on
Hmm....just seen that haibike price has shot up and the £ very good against euro as well......