If you haven't found anything like this already, I would recommend following this blog post
http://chris.kehayias.com/2011/07/10/complete-guide-to-streaming-live-video-to-roku-part-1/. The demo app he builds shows you how to have multiple categories with multiple streams, as for switching things around automatically, I am not sure, but this could get you off to a good start.
I can't comment on the Cost of hosting on Amazon. We self host Wowza and paid once for the server and the Wowza perpetual license (they do a non-profit discount if you ask them about it).