Create own video course web platform

I am considering building an own online video web platform/portal, similar to lynda and video2brain, having following assumptions:

• 10K to 100K users (students) that can watch online videos, divided into different courses.


• Courses divided by up to 10 groups/categories, that uses can subscribe


• Users should be able to login and make annual subscription or buy individual courses


• Video upload will be centralized - I want to make the videos and I will upload


• The users might use tablets and smartphones to watch, so responsive web design might be important


• The language of the portal will be english, localization might be an option but then a long-term goal

I can build not too complex web sites, I know HTML5, CSS, Javascript (jQuery), Wordpress, Joomla, also Java (SE 8), many DBMS (like PostgreSQL, MySQL).

So, what should I consider when building such a "heavy duty" web platform? Is it possible to build it completely using open source software?

Or is it worth to use own software/hardware or use external services? What montly costs should I consider?

Open to all suggestions / opinions!!!