18 hours layover. Do I need to take my stove luggage if I sleep at a hotel outside the airport?

Filippo, if you are traveling on a single ticket (single booking), you do not have to bother with your checked luggage because it will remain at the airport and the airport staff will take care that it is transferred from a flight from Italy to the flight to Australia. You can be perfectly calm in this regard.

As far as your exit from the airport is concerned, you do not need 24 or any other visa free transit stuff because Italian citizens can enter China absolutely visa free for a period of 15 days. In other words, upon your landing in Shanghai, you will just fill out the arrival/departure card (they will be dispensed on board the plane before your landing as well) and just step in the immigration queue to exit the airport. In other words, the entire procedure with your entrance into China for you is as if you were traveling to London, for example...