Hmmm, not sure how helpful I can be but here goes:
1) Not sure what you mean by self sufficient? Equipment-wise, if these plants grew in this setup then they should continue to do so unless you add new plants with different requirements.
2) Definitely worth keeping plants. Keep what you like, add new things etc. It's all up to you. Depending on your changes you may have to adjust the setup (lights, etc).
3) Do you mean fertilizers? Potentially. Depends on what your plans are (see #2). I personally don't use any, but I also don't have rapidly growing high-demand plants and I am lazy.
4) You can...gravel attachment on water change siphon works well. I personally wouldn't go too OCD clean on that though...and it doesn't hurt the plants to have nutrients in the gravel.
Methinks you should envision what you want it to look like, and then figure out how to get there.