A couple of points from our experience in this process...
1. Get the owner to do the de-registration with AMSA before the transfer of title. You, as a non Australian citizen, can not own a registered vessel.
2. Retain the Queensland
rego. This will allow you to continue to sail the boat until your US rego (either state or Coast Guard) becomes valid. You will not be able to clear out of Australia until the new rego is valid (this was a real nuisance for us!).
3. You will not be burdened with either GST or duty if t he boat has been built in or imported into Australia. Further, you will not need a cruising permit
while sailing in Australian waters, and this is a real advantage!
Enjoy your cruising here... we surely have, and we've been doing it for a long time now.