Aquasoil is a great substrate and you don't need to have anything else in order to have an "Aquasoil System." A lot of people use Aquasoil only. Powersand is not necessary at all, in fact some people have actually commented that Powersand can cause problems and I don't recommend you use it yet. I highly recommend Aquasoil only.
Go to Aqua Design Amano USA/ADGshop.com- now available in the U.S.A, prouldy offered by Aquarium Design Group. The finest planted aquarium products.
and check out the substrate section. Click on the Aquasoil type you want (Amazonia/Malaya/Afrikana) and then click on the "extended information" tab. That will tell you how much aquasoil you will need. You can also call ADG and speak to them and ask questions, they will help you out a lot. Tell them "wood" says hi
They are good people and have tons of knowledge.
Good luck with Aquasoil you will love it, it IS the best.