Hi Denis- I'm a BIG FAN of teachers who Dream Big for their students and want to improve their education.  So my first response is YES- You Can Do Anything!  KA is perfect for multi-grade and varied ability students.  As a coach, it will be alot to keep track of and you will learn as you go.  As to making videos - I also think this is a completely attainable dream.  There are lots applications that make it easy to create videos. FInally I would add these two things:  #1- my students still get a lot of direct instruction from from me (I don't think that is a bad thing at all) and #2- the videos on Khan Academy are really good.  I'm impressed witht he level of explanation and depth of understanding they show.  So when you just feel stretched too thin to create a video- the KA videos are great.

Good luck and congratulations for Dreaming Big.  Your students are blessed to have you as their teacher!