Ok, so the 120GB ssd is your boot drive and the single 500 is for storage? If this is just for testing that is fine but if you are going to need more storage later on yo should get it now, Freenas storage is not easily expandable, plus you may want some redundancy.
First thing to consider is that your storage and jails are separate, before you even start with jails you should go ahead and create datasets for storage. For example, create a dataset called "storage" and in that dataset create another called "dvr".
You can set permissions for each dataset individually, you should read up on permissions and understand them fully, I'm not sure what the default user is when creating the Channels Jail from the plugin, but when you manually create the Jail using my method the User/Group is Channels/Channels and the id is 820.
Honestly, Freenas isn't the easiest OS to jump into, you're going to have to do alot more reading if you want to understand what you're doing and why it is or isn't working. For your use case you may be better off with a linux distro or Unraid.
Here is a look at my datasets, there are also additional datasets under "apps" in which I store the config files for my other jails, that way I can destroy the jails without deleting the config, which makes it easy to reinstall. Channels doesn't need the config stored outside of the jail because it can be reinstalled using the files located in your "dvr" dataset. Again, the "dvr" dataset used for storage will live OUTSIDE of your jail.