A new water heater means no more lukewarm showers or worrying about a flood in the utility room. Most swaps take about three to four hours from start to finish. Booking in the early fall is usually cheaper since plumbers aren't yet buried in emergency freeze calls.
We want to avoid the rush of an emergency repair. Getting three quotes is the only way to know you aren't overpaying for labor. You have more leverage when your current heater is still technically working.
The 0% window is a firm deadline, not free money. If you still have a balance after the 18 months are up, the standard interest rate will apply to whatever is left, which adds up quickly.