For serious off road a mid-drive is best, but not necessary for light off-road. 16 st is quite heavy, so you certainly need to keep an eye on rear wheel spokes anyway, and especially for rear wheel hub or significant off-road. Mid drive will get through chains and sprockets much faster.
If you want to keep legal you need a motor that is nominally 250w (or less). It should still be able to help you up hills, and beefier ones generate up to 750w peak. Max legal assist speed is 15.5 mph.
What matters on battery capacity is Wh (watt hours) which is ah (amp hours) * volts. So if you have a 48v system you need less ah in the battery for the same capacity. A 500wh battery should be able to do 50 miles, but depends how you ride. 600wh would be safer. If you look at
https://www.bosch-ebike.com/en/service/range-assistant/ you will get a good idea of how far you can go depending on battery size, your weight and riding style, and different road conditions. You can find differences of more than a factor of three (100 miles or 30 miles) for the same battery depending on those details.
15ah Soundwave suggested is 540wh at 36v, or 720wh at 48v.