Mosser, I also applaud you for "getting it," and also getting that learning & implementing GTD is a project, and that you need to start where you are.
One thing I will comment on is the In Box. Having only one is a great idea in theory, but in practice, I think you'll find you will have - and, indeed, need - several. For example:
- voice mail
- email in box (maybe multiple depending on how many email addresses you have)
- paper inbox at office
- paper inbox at home
Even though you intend to use an electronic list manager, paper will still creep into your life. While I don't have ADD/ADHD, I know plenty of people who do, and I'm guessing that you will experience great freedom in being able to have these random ideas you have and capture them - either in voice memo, on paper, or whatever - and just dump them into your inbox for later processing. It will free up your mind and yet you can process them when you're in a more focused frame of mind and able to go through the processing questions on each item. (A lot of ideas you might just collect in a "shoe box" - Evernote, or wherever - "Ideas I May or May Not Do Anything About"...)
Just my two cents. Good luck!