Late summer is the best time to start this project. As the weather cools and retailers clear out lawn equipment, you can find the materials you need for a better price.
You are looking at about $300 for a decent pit and rubber border kit. Planning for this now keeps the cost manageable and avoids a hit to your monthly budget.
Remember that the 0% intro period is a deadline, not free money. If you have a balance left after the 18 months are up, you will start paying interest on whatever remains.