Hello everyone !
Im looking to travel to Australia from end of April with as less spending as possible. To do so, I know Ill have to sleep in hostels. Ive researched a bit and found out hostels are about 50CAD/AUD/per night in Brisbane/Sydney/Melbourne (correct me if its wrong). I hope to spend less than 2500$ (flight and accommodations excluded). I am overwhelmed with questions right now so Im just going to list them here:

Is it too cold for beach days in May? Where are the best beaches (those that are not overcrowded with tourists) in Aus?
What are the must-go cities? How much time should I dedicate to each one of them?
Any tips on how to eat cheaply there? I know hostels have kitchens but I dont really see myself staying there till lunch time nor going back there for supper. Is it a good idea to rent a car to move from city to city (say Sydney to Melbourne or Sydney to Brisbane)?
What are the must-do non-touristic activities?
Anything I need to know about travelling to Australia?
Is it a good idea to visit isolated locations and beaches by myself safety and logistically wise? How do people usually get to those isolated places?
What should I be cautious of?

Thank you folks for the help. If you guys need any help concerning Canada and especially Quebec, please dont hesitate to contact me.