A new exhaust system changes how your car breathes and how it sounds on the road. It is a solid upgrade if you enjoy driving, but it is not a small expense when you factor in the parts and the shop labor to get it installed right.
Wait for the end-of-year sales cycle when shops are clearing out inventory to hit their annual targets. You can often find the best prices in December when performance parts retailers are trying to move stock before the new year.
Remember that this is a repayment plan, not free money. If you have any balance left after the 18 months are up, you will start paying interest on what you still owe, which makes the whole project more expensive.