A few comments, with questions
- Please say what you really want. You say you want integration of tasks and calendar, but you also mention handling of projects without saying what you are looking for. Also, what platforms do you use? I would guess iPhone and Mac, but I can't be sure.
- Integration of calendar and task list is increasingly rare. Busycal does a good job with Calendar and Reminders on a Mac, but is not going to give you any project linking or hierarchy. I don't care for Pocket Informant, but it does give you a unified view of calendar and next actions on an iOS device. You might like PlanBe better, but it uses data from Calendar and Reminders.
- If you want a robust hierarchical list manager on OS X and iOS with a view that shows your calendar, then you have one choice: OmniFocus. It's a big, relatively expensive, sophisticated piece of software. You may or may not be happy with it.
- I don't know what you mean by a "date limit."
- I think the easiest, best thing for someone starting out with GTD is to do exactly what David Allen tells you to do, in his book and in the various set-up guides. In particular, I would give up on Calendar/Next Action integration. It's not worth that much. If you need to do an errand like going to the bank on a specific date, you can put it on your calendar.