we had a treehouse before, it would have been good for growing weed too lol, it took about 10 of us three days to make, it had three floors, a hammock and a window, its was the best treehouse ive ever seen, then it got ripped down by a bunch of pricks... so its a good idea doing it on your own land..

sounds like you've got a good plan at the moment, dont forget to add worm castings to your medium, they are super-beneficial, so its worth buying some if you cant 'make' some... - good luck...