One thing I've noted is that no matter how well you think you have the system down, GTD is like an onion, you keep peeling back layers of understanding. SO just because you want the system to be complete at the start you need to get comfortable that your understanding of and use of GTD will grow over time and don't beat yourself up about whether or not you are complete. Just jump in and get started and refine it a couple of months down the road.
As a former boss used to say "Ready, Fire, Aim"
Or in a slightly more verbose version:
"Continuous improvement after seeing results is more important than a perfect shot
and sometimes laying down an unfocused barrage is
enough to get you the time to make the next shot count."
Works for everything from fire fights, to programming, to dealing with recalcitrant sheep.
Do you think that i have made a good start with my implementation in my first post ?