No problems with transit at sydney, just tell them to follow the signs and they'll go through a security check and go straight throug to the departures section of the international terminal

Yes, they will have access to the international business lounge (they have no physical access to the domestic lounge unless they clear customs). More interesting is singapore, the access rules are that if you are flying business class that day on QF you have access to a qantas club business lounge. Singapore has a qantas club so if the flights are on the same day by the letter of the rules they have access there as well (by my reading of the rules but they'll want to be able to prove it at the lounge)