Assuming that you have cable brakes, get the in-line brake sensors rather than the levers:
HWBS - Hidden Wire Brake Sensor 1pcs - BMSBATTERY
Torque arms are advisable.
You need to order the LCD and wheel speed sensors because they don't come with the controller.
You also need to order a pedal sensor.
It would also be a good idea to get a thumb throttle as well just in case the half-throttle doesn't fit.. It's always useful as a spare and only costs about £2. If ever you have to buy another one later, it'll cost about £10 to £15.
The battery is just about on the limit for current. Personally, I'd go for the 20aH shrinktube one and install it like the photo below unless I was going to frequently park the bike in a public place for long periods. I cable tie the zip to make it tamper-proof for short stays at the supermarket or pub.
36V 20Ah Li-Ion Shrink Tube EBike Battery Pack - BMSBATTERY
Waterproofness depends on your installation. The motor is waterproof, but the throttle on any bike is not 100%. If left out in fierce rain, water could find its way in. LCDs on any bike are also not always as waterproof as they should be. If I was going to park my bike in the rain, I'd cover the battery, throttle and LCD. Riding in a shower isn't a problem.
If you do install your battery like this, it's totally waterproof. The bag costs about a fiver, and the rack about a tenner. The charge socket is underneath, so it never has to be unzipped.