Owning a car means dealing with heavy maintenance bills before things break and leave you stranded. A timing belt and water pump service usually costs about $900, which is a massive hit to your checking account all at once. The best time to book this is when shops are slow, typically in the dead of winter or mid-summer, and you can often get a discount if you bundle it with a simple tire rotation.
Never let the shop finance the repair for you, and keep negotiations about the price completely separate from how you plan to pay. Once you lock in the lowest labor rate, you can figure out the best way to spread out the cash without draining your emergency fund.
Remember that the intro window is a deadline, not free money, and what happens if the balance is still there at the end is that you will get hit with all the deferred interest. Pay off the full $900 before the 18 months run out so this repair stays cheap.