I'm working on a script that will need to access several google calendars (all under one user, always the same calendars) and retrieve appointment data, delete appointments and create appointments.
Would a google API be the best tool for this, or am I looking for something else?
I'm not sure I understand your question.
If you are accessing an external website for data, you pretty much have to use the API they give you. The API itself isn't a tool, its simply the mechanism in which you are allowed to talk to it. How you get there is up to you.
If they have a soap protocol api, that's what I'd use despite it being somewhat difficult to use properly in PHP (datatype weakness really is a weakness).