Can't agree with Terry & Paul more.  You MUST have this drawn up correctly for your protection.

You will require an Option Agreement or a Licence to Occupy, RTA lease form 18a, current residential contract (for unit under body corp specific).  There will be other paperwork as well to put in place & it is imperative that you get it right.  These are the bits of paperwork that if things go sour can save your bacon in a small claims court.