Cars are basically just a series of parts waiting to fail. The battery and alternator are the worst because they often wait for a freezing morning to quit on you. Replacing them now means you avoid a tow bill and a morning spent shivering on the shoulder of the highway.
Mechanics are usually less busy in the late fall before the winter rush hits. If you book the service now, you can often talk them down on the labor rate since they want to fill their bays. For a typical $600 bill, we can use a specific timeline to break that cost into small pieces.
The 0% interest window is a fixed deadline, not free money. If you still have a balance after the 15 months, the standard interest rate applies to whatever is left. Pay the full balance before the clock runs out to keep the repair cheap.