Good idea with the license.txt.
I like the idea of keeping it simple.
I'm of the opinion that downloading the necessary files as they are required would be the best approach.
I think it would be easier to maintain that way.
Like I said, this would be maintain licensing as you wouldn't be actually distributing it as part of an open source package.
That way you would be able to release the whole thing as open source without including any third party licenses.
It's not mission critical though, but worth thinking about.