Where should we buy an international SIM card?

What you could do is buy a SIM card from the Sim Corner outlet in Sydney's International Airport. They can fix you up with a prepaid AT&T card which offers excellent coverage all over the USA and great rates. Sim Corner also has other SIM cards with phone and data plans if you plan to be a heavy user. For example you can have a SIM card that gives you unlimited data, local calls, texts and incoming calls for $129 and that's a great price but if you can rely on Wi-Fi services at your hotels it's probably not necessary. You'll find Sim Corner in the airside part of Sydney Airport's international terminal, after you pass through immigration and security.

Bakeries and supermarkets are everywhere, nobody goes hungry in America if they have a dime in their pocket. America invented takeaway food. 

How much money is a tough question. On a tight budget, you could travel, eat and sleep for $US100 per person per day, but a reasonable budget is US$200-$250 per person daily. Don't carry a whole lot of cash around – just about everyone takes credit cards – and if you have a travel money card, you have the convenience of withdrawing cash from ATMs when you need it, although you will pay a fee for every withdrawal.

Hiring a car is a great way to go in the US, but only if you're travelling around and seeing the sights. If most of your travels involve cities, then a car will be more trouble than it's worth.

Taxi drivers in the US are no more inclined to rip you off than taxi drivers anywhere else, and that's to say very little.