Regarding places to park, check out Park4night app in your phone/tablet or go to https://park4night.com/
There seems to be plenty of places listed in Australia too.
I saw a UK video on youtube where they said there is a UK company that acts as your PO box. It receives all your mail and packages, then resends them to whatever address you want. If there is a similar Australian service, it might be a valid option.
Another option is a trusted family member or friend who would do that for you.
Regarding costs in general, you should examine what kind of vans/platforms are common in your area. If you know anyone who does car repairs, ask they have certain models they would recommend or what to avoid. Some certain brand/model could be really tempting and cheap to buy but parts would be much more expensive or harder to get compared to another brand.