If you need your transport directly from the airport I found that if you go into the car rentals sections, you would often be able to haggle price as some of the rental companies are more desperate than others and they will sometimes shout over each other from different counters offering you a better deal.
Take a look online. Sometimes there are some benefits to prebooking online. Have a peak at the rentals online and see what discounts may be available.
Insurance is an issue and very expensive. Could easily add 30-80% to the cost of your rental. Make sure to budget for that, since you need it. It is the law in Canada that you must have at least 3rd party liability insurance, plus you do not want to be landing in a new country with those kinds of issues.
My best suggestion is that if you are planning to buy a car, do it as soon as you possibly can. Will save you loads of money. More often that not you can walk into a dealership and walk out with a car. Not hard. Most of the decent dealerships also have websites, so you can easily take a peak and plan for that. And chances are even if you just buy something to get around for a short while, (a clunker), you could end up spending less on that than a months rental would cost. And you can sell it again. Probably get a lower price but still worth the trouble.
Good luck
