1) In a close loop system, the transfer function in Simulink behaves as the "plant", which is essentially the quadcopter actuators - I am not sure if that's completely correct. When you say 'plant' it includes everything right from your sensors, actuators and the quadcopter itself. The controller commands the actuators, the quadcopter responds and you get the output data/states from your sensors which are used to close the control loop again via the controller.
2) Is there any particular transfer functions that model closely their behaviour ?- What is 'their behaviour' here? There are generic models of sensors and actuators in simulink which you can tune and use There are couple of DOF demo models. Also you can give a check on file exchange. Pretty sure you'll find some flight control models there.
3) Is the aerospace blockset helpful for such modelling and simulation purposes?- I would say yes but again it depends on what exacly you are trying to do. Give a quick read about aerospace blockset here and decide for yourself.
