You're covered under the dealer's policy when test driving; if it's a private owner and he has insurance on the car, you're also covered by that to test drive. (If he doesn't, don't drive it.)
When you've decided on a car, you probably should call your parents' agent (if you're under 18, you have to, since you can't get your own) and ask for a premium quote. The type and age of car won't make much, if any, difference UNLESS you have to carry comp/collision coverage, but if you can pay cash for a beater, all cars will cost the same, provided it's not a hot sports type. Same thing if you use one of the web sites--just plug in a 'dummy' make and model for figures. If it asks for specifics, like a VIN, go on to another site, but at your age (16?), your age will be the governing factor. Once the agent binds you, or adds you to the parents' policy, you're covered to drive the car home.