You can decide for yourself what purchasing "model" you want to use if you are creating your own purchasing system.
If you are using the Channel Store payment system instead of your own, you can have one-time purchase of all content, or, monthly or yearly subscription purchase of all content. There may be other options in the near future, but I don't think they are geared towards multiple purchases within one channel from the Channel Store. For PPV or video episode purchase, you will need to create your own subscription / purchasing system.
- Joel