It looks like the battery comes out at the bottom bracket. I'd like to see what's down there. There was another one like that, which we were looking at, that had a lock down there, which is surely going to get full of crud and jam.
With these sort of bikes, you have to think about the future. The chance of getting spares, like a new controller, in 4 years time is close to zero. If you're good at electrics, there's always ways to fit different batteries and controllers, so it shouldn't be a problem unless you can't do that, in which case you'll have a nice new trellis for your peas and beans.
One other thing. If you want assistance above 15'5 mph, make sure that youn know whether it's possible and how to do it before you buy that bike, otherwise think about one that has a known solution.