I want to stream using AAC, do I have to buy Wirecast or another encoder?

First, if you have a Mac, you will have AAC on their already for use with Adobe Flash Media Live Encoder.

If you are on a PC, then you can buy a AAC codec for $180:

Otherwise, encoders like Wirecast are the best solution in order to get the ability to broadcast using the AAC audio codec. Wirecast has a free trial for their encoder as well, allowing you to sample what your live streaming will be like using AAC.