You are buying a machine to pull real shots and steam milk at home for the next decade, which means you want to buy once instead of upgrading twice. A proper setup changes your morning routine and pays for itself over years of skipping the cafe line.
Holiday sales are the right time to pull the trigger because manufacturers and kitchen stores drop prices to clear inventory before the new year. If you watch the release cycles, you will often find that last year's model gets the steep discount while having the exact same brew group and steam pressure as the fresh box.
Remember that the intro window is a deadline, not free money, and the remaining balance will start accruing standard interest if you do not pay off the machine before those 18 months run out.