Does the 900 Liter Reservoir Bag come with the mask and hoses needed? Or is there anything else I need to add?

The 900 L Bag System comes with everything you need - Bag + Mask + Accessories. Optionally you can purchase the Bag Stand. It's handy, portable, and highly recommended. Also, if you are considering an additional mask, this is the right time to buy as you will save on the cost + shipping.