You're right in targeting the US market for your ebooks. That's what I did when I started out, and have found that about 80% or more of my sales still come from the States. You're also right in assuming that they are fairly US centric. They can afford to be, being such a large country and dominant player in the world. Australia and New Zealand, on the other hand, are not. So we have become used to accepting sources and information from the US and Europe without batting an eye.
So, to answer your question, I would suggest that you try to make your information, writing style and even English usage as international as possible. And, by international, I really mean US! You will have noticed that The Wealthy Writer is written this way, even down to the wording (eg color instead of colour, check instead of cheque).
Having said that, I am not suggesting that you do not have any NZ quotes, comments or info - I am just saying that you should make the information and advice as international as possible. Again, consider my book. All of the clients quoted in the book are Australian, all of the examples cited are Australian - but the information, the philosophy, the formulas for success are international. That's why I have readers all over the world - including unlikely locales, such as Kenya, Japan and Russia. I also cite US examples where possible - and this is something you can do, either through online research or at your local library.
As for Google adwords, I cannot say. When I wrote my first ebook, I deliberately chose not to do any 'above the line' advertising or marketing. The only promotion I did was online - developing several web sites and ezines, contributing to a range of forums, etc. Whatever you choose to do, I would suggest you start with these fundamentals which are outlined in detail in my book. I think you'll find them far more useful and far cheaper than advertising.