As noted you must pay for these limos and it is actually less expensive to get one from the airport (self-booked).

As for early arrival at Sydney, it really depends on how booked they are and if you want your upgrade. In the past as an AMBASSADOR I've been offered a room to clean up in for an hour, checked my bags, and then returned later in the day to get my room. But again, it all depends upon how full the hotel is. (This was a few years ago, pre-RA, so this might not be made available to AMBASSADORs any longer.)