We bought this property in Melbourne and the lawn was a mess. Previous owners didn't care for it and it had more weeds than grass and uneven patches. It seems like whoever laid the turf didn't join the rolls together.
I went nuclear on it by applying Roundup and now everything is dead. There was also some gravel at the end of the backyard that I have removed.

Should I remove the existing dead grass with turf cutter or use the rotary hoe to cut the grass into small pieces and use it as compost for the new turf? Can a cultivator (Ozito cordless cultivator) or tiller be used for that purpose?
Do I need to place ag pipes for drainage or the rain water can flow under ground (apologies if I am being naive)?
Do I need to add top soil? If yes then how deep?
Lawn is 8m x 2.4m.