Hi Ram,
Hope you are doing great !
To begin with, I must appreciate the way, you have explained points. I would like to bring to your notice that we too are working on education portal, which is clone of Udemy.com.
Due to NDA I cannot share the link but, I can surely provide you the design files, for you to review the same.
As far as the project is concerned, we can definitely manage the test series functionality to the existing WP system, but keeping the complexity of applications and managing number of elements, I would suggest to recreate the website using PHP based MVC framework for better performance and management.
I have mentioned a ball park estimate at the moment.
*Rest assured Time & price will not be a deal breaking factor between us.
I can start off as soon as I am being awarded the task.
Anticipating your positive response.
Regards,
WPOPPO