For car rentals in Sydney you can try the price comparison website with by far the best name ever VroomVroomVroom.com.au - I know Vroom is often thought as Virtual Room but in this case it is vroom, vroom
Compare Sydney Car Hire from Avis, Budget, Europcar, Hertz, Thrifty
Just checking a few prices from pick up and drop off at Sydney airport and this site Discount Car Rental Sydney™ | For Cheap Car & Truck hire Sydney Australia offers a Toyota Corolla rental for $39 per day or a 8 seat mini van for $89 per day.
At $39 a day that's cheaper than 4 people getting from the airport to central on the train. For 4 people the trip from Sydney Airport to Central train station would be $50.40!! However with the hire car you've gotta think do you have free parking at your hotel and cost of petrol or diesel.
Will have to look into luxury and do a few more tests on prices to see if we can find the cheapest car hire for you.