All you really need is a seed for your live sand. Your best option is to find a couple of people with well established systems and get a pound or two of sand from them as a seed. The more people the better because this will add to the biodiversity of yout sand bed. In my opinion this is better than buying a bag of live sand that was bagged up months ago. If you must use bagged live sand then just one bag would be enough to get you started.
Here is a good article by Dr. Ron on deep sand bed particle size and the critters that live in them. -
http://www.ronshimek.com/Deep%20Sand%20Beds.htm
Since the base rock is essentially a clean, uncolonized surface remember to add plenty of grazers like various snails and crabs to help keep the algae in check until the tank becomes established.