It will take you at least an hour to pass through customs. A taxi may cost about $50-$60 into the city but peak hour is 7 am - 9 am, Uber would be cheaper. I always catch taxis straight away but they by pass the city. Lately we have caught the Skybus to the city. The Skybus is easy to catch and about $20 and leaves every 10 minutes. You catch it as you exit the Arrivals Terminal doors, then turn right. The bus is a short walk and departs outside Domestic Departures. You can catch a taxi to your hotel from Southern Cross Station where the Skybus journey ends. Ask the driver where to go to catch a taxi. It would be a short taxi ride to your hotel - may-be about $10-$15.
To go to the port on Saturday catch an Uber or taxi to Port Melbourne. It will probably take about 30 minutes on Saturday late morning for about $40.
Most times overseas, we have been given access to our rooms earlier than the afternoon. You will be tired after your 24 hr flight but If you cannot get into your room, by all means take the free tram around the city. You could also catch a hop on hop off bus - it goes down to the bay area and is a pleasant ride. I think you catch the bus at Federation Square which is a 10-15 min walk from your hotel.
Good luck and safe journey.