If you are using a Dynamic Plugin, you can set the trigger right in the dialog box.
You first define the trigger variable as $_SESSION['userlevel'] and then hit the + sign to add it. Then select a plug in source and then enter the level you want in the variable value box. That way it will only pull the plug in if the trigger variable is equal to the value you enter.
I think you would only need to check for the userlevel session variable because that would also prove that the person is logged in as the only way it could get set was if they successfully logged in.