Technically you need to be insured when you get behind the wheel. When buying a car from a dealership, you are required to have insurance before you drive off the lot. Since you are buying the vehicle from a third party, you will, at least, need to insure the vehicle before you register it. You don't have to purchase a policy before you buy the vehicle, but if you get pulled over on the way home after buying the car you could be ticketed for being uninsured. If you were to be involved in an accident without coverage you would be held responsible for all the damage you caused. I would recommend that you shop for insurance
once you have decided to buy the vehicle and start a policy as quickly as possible to avoid any issues.