Trying to be as objective as I can, given that you want to make three complex extensions (though there is an add-in mod for Zen Cart that does the third, if it does it the way you want to do it), which makes the most sense ...
  1. Extend a dedicated e-commerce application or
  2. Extend something that in turn is already an add-on to another application

Also, given that complexity, you shouldn't be looking for a coder in the first instance. You really need a business analyst who can tie your requirements down accurately before any coding begins.