Yes, you can update this phone. However, this phone comes with latest Android 7.0 Nougat out of the box. So, you may not require to update the phone immediately.
No. Moto G5 does not have front camera flash but the display flash is there, which is as powerful as LED flash.
Moto G5 has Water-repellent nano-coating.
Moto G5 3GB RAM dimensions are 144.3 x 73 x 9.5 mm.