18l of amazonia is enough to form a decent layer of substrate, although if you intend to grow bigger plants you might want to put in a third bag (AS tends to compact overtime, bout a cm IME).
Powersand is not a must. and i wouldn't recommend it if this is your first tank or if you will be changing landscapes often. Other sand-type nutrient base such as those form JBL or Dennerle etc. will work well too.