Having a deep-freezer in the garage means you stop running to the store every time you need a bag of frozen peas. You can buy meat on sale and actually have the space to store it, which saves you a ton over the long run.
Wait until the fall to shop, when stores rotate inventory and clear out old floor models. Getting a few quotes from local appliance shops before you pull the trigger will help you find the best price on a reliable unit.
Remember that the 0% window is a deadline, not free money. If you have any balance left after the 18 months, the bank will start charging you interest on the remaining amount, which will cost you more than the freezer is worth.