I found it cheaper on two trips to the USA to arrange car hire from here when booking my flights. I travelled through Trailfinders of Dawson St.
I would not skimp on insurance. Not obligatory to be fully insured in the States, but, worth the money for peace of mind. Someone on the forum who workes in insurance will be able to give the technical details, but, if you are in an accident and not fully insured, charges... monetary and legal... can quickly mount!