Hi,
You have a very ambitious project. I see that you want to create 2 websites, one on WordPress and one on Moodle and integrate them to sell courses.
Edwiser Bridge will solve that problem for you. It was developed specifically for adding this functionality to Moodle.
When students to log in to Wordpress they should get logged in to Moodle website simultaneously, that can be achieved using Edwiser extension Single Sign-On.
Using Single Sign-On, you will achieve what you need. Your students can stay on your WordPress website and will be redirected to Moodle only when they buy the course/click on the course.
When students buy courses on Wordpress their ID will be automatically generated in Moodle too and using that ID they can access both Wordpress and Moodle.
I hope this information helps you in deciding.