Hi,
ADL will provide headsets but they cheapo DC copies which are about as old as the duchess, your own ones would be better. On the aircraft CX and sims the headsets are ok probably not worth your while to buy your own. The amount you will use a heaset as a SO is hardly ever, maybe to keep your hair in place after you get out from the bunks.
There are no 777 SO on line at the moment but i think the roster will not be as comfortable as the 400/340 guys and girls. You will mainly do 3 crew trips (SO always get crap rest and works the most on 3 crew) and the layovers and patterns will generally be shorter therefore you may have 4 trips/mth as oppose to 3/mth. 777 at the moment generally middle east SO as a legstretch pilot.
Have fun, get ready to say hello to Dubai, bahrain & Riyadh. You can forget the rest of the network for now.