Hey there,
I'm currently in Australia on GROW, and I completely understand how frustrating getting here is haha.
I heard back May 25th, so if they're running late again this year that will be tough for you to get organized in time unless you start before you hear back. NSF gave me a contact with Universities Australia to help out, but they made it clear that they were pretty hands-off on helping people through the program. I was told that they would prefer if I travelled with a Training and Research Visa (subclass 402, occupational trainee stream), but that it would be possible to travel on the Work and Holiday Visa (462) if time was an issue. I wasn't planning on leaving quickly so I went with the 402, but that was a huge headache. I applied for it in early June and didn't hear back on it until late October, which was about a week away from my departure date. In that regard, I would definitely go with the 462 visa (if you're under 30) because it is way less expensive and is processed and approved instantly. Also, my Universities Australia contact told me to purchase Overseas Student Health Cover, but this is inaccurate information if you're not traveling on a student visa. You actually need Overseas Visitor Health Cover (unless your home insurance covers you abroad).
As far as housing goes, I arranged to rent a room on Airbnb for my first two weeks and found an apartment when I got here. That might also be a good plan for you, especially if you found a place with a flexible cancellation policy. You probably wouldn't lose much money if you didn't get to go. I was glad I chose to do that because I got the chance to check out neighborhoods before I settled on a place. It's probably more of a toss up on what to do for flights, but if you don't need much of the travel allowance I guess you could probably afford to pay more on a flight from having to book not very far in advance.
The travel allowance was put into an account for me at my university. An administrator was able to use the money to pay for my flights upfront so that I didn't have to wait to get reimbursed, but other costs will be reimbursed for me. It probably depends on your university's policy. I'm not sure about whether they'll give you 5k anyway, I asked for the full amount.
Hope this helps, let me know if you want more info about anything.