Hello there and welcome to the Wowza support forum.
You might want to start with a Trial License to get started. After that you could purchase a monthly license or, depending on your long term needs purchase a perpetual license.
Wowza comes with great features out of the box, there are also amazing free AddOns that you can enable. The list of premium AddOns(purchased AddOns) can be found here:
Premium AddOns
A free AddOn that has recently been incorporated into the initial software package that you may find valuable is the MediaCache System:
How to scale video on demand streaming with Media Cache
At present the Wowza Transcoder does not support transcoding of VOD, so using the Image Overlay feature would not work.
There is a work around of using the VOD files as the source for the live stream, and running that stream through the Transcoder, then re-recording the file with an overlay.
I hope this helps get you started. Please let us know how we can assist you in the future.
Thanks.
Salvadore