Last week, an official take on GTD with Asana was published:
In GTD, something is a project if it takes multiple steps to reach the goal. So setting up an appointment qualifies. Personally, I wouldn’t create a project or subtasks for a project of which the sequential steps are completely predictable. Just rename the task to the next step.
For Waiting for, I use Asana’s Tag feature. I also tag tasks that need to happen at the office (the rest I can do pretty much anywhere).
For stuff you handle via Outlook/email, it’s not weird to keep it over there. Just make sure to check up every once in a while. For most people, a weekly review is sufficient. So you could have an email folder “Waiting for” to which you copy all email messages like order confirmations, requests for calendar dates and documents you asked for. Then move the email to a folder “Archive” so every email you’d want to search for is in that folder. During weekly review, check the “Waiting for” folder, delete what’s been done and decide whether you want to act on stuff you didn’t get yet.