…oder wie man sich möglichst komplex ärgert…

so… I’m in the process of selecting the right GTD (Get Things Done) Software for me. There are some features that it needs to have:
1. Online Storage. Need this to be accessed from anywhere, so it must either live on my rootserver, or on some provided Online storage.
2. iPad App. Got to access this from my iPad, so it needs a corresponding App. Browser usually is not the best choice here.
3. Structure. It needs to have some kind of “Folders”, where stuff can be put.
4. Sharing of Objects. Sharing must be possible for single Objects, or Folders. It must be shareable with specific persons, and it must be free for those persons to access. I am willing to pay for my account, but not for several.
5. Nice-to-have-Option: Application for PC and Mac. If the browser interface is nice enough, this can be omitted.

Currently I use SpringPad, which has two major drawbacks: I can only share items with the whole world, not with specific users. Structure is bad for me, since every note ends up in a collective Folder, even if I put it into another notebook, it stays in the main one.