High-performance cars are a blast until you need new rubber. Sticky tires wear out fast and cost a lot. You are usually looking at a grand to get back that cornering confidence. It is a necessary cost of owning a fun car, but it doesn't have to ruin your monthly budget.
Buy in the winter. Shops are slow and manufacturers run heavy rebates to move inventory before the spring rush. This is when we haggle on the price of the tires themselves before we worry about the financing. Negotiating the price separately keeps the math simple.
The 0% interest period is a tool, not a gift. If you still have a balance after the 18 months, you will get hit with the standard high APR on whatever is left. Stick to the monthly plan to make sure the tires are paid off before the interest starts.