I was sorry to see that no one has responded to your post as I am planning very much the same project.
After researching many large planter ideas online, and realizing how expensive doing a brick or stone or cedar/redwood planter of the size I'm contemplating would be (not to mention the amount of work and expertise needed to get it just right), I am now thinking I'm going to go for the concrete-block-with-a-finished-face approach.
Like you, I am trying to figure out the foundation, too. I'm asking myself these questions and looking online for answers: should I just build it on the patio, or should I put it in the dirt alongside the patio? If I put it alongside, couldn't I simply sink a row of concrete blocks all around as the foundation, then do the fastening and strengthening as I build the planter by using rebar and concrete in some of the holes in the blocks? (I learned that last part online!) If I do it that way, would I eventually have problems with settling, shifting, etc, that would make the planter crack?
I intend mine to be 30" to 36" tall, about as wide, and 7 feet long, so I will need some vertical stabilization. But I don't want to get too carried away and make it so hard I never get to it!
Have you had any other ideas or advice about your planter?