The GST is based on the exchange rate the bird leaves the mainland or in the case of the US, Hawaii. All the fuel AND accommodation you purchase on your merry way (Christmas Is, W/Samoa/Fiji etc will also incur the wrath of the GST. it would be strongly advisable to register as a business with GST reporting B4 you buy. You could always cancel at a later date.
Yes, i would definitely take my engineer next time as the Yanks are extremely slack with aircraft maintenance, especially AD compliance. The "mechanics' over there always seem to be sitting on their acres while the meter ticks over at $US 70/80 per hour. What is written in the logs is fairytale stuff ie AD's signed off but not done.
and unreported damage, often major.
There are a few traps for the uninitiated but its all possible.