You want a camera that lasts, not a toy you will replace in a year. When you hunt for a high-resolution mirrorless body and kit lens, look for gear that is two or three years old. These models are still professional-grade and hold up perfectly for years of shooting.
Wait for the brand to drop a new version of your target camera. The moment the new one hits the shelves, the price on the previous generation typically drops because stores want to clear out their inventory. You get the same quality for much less.
That 18-month window is a deadline, not a gift. If you have a remaining balance after the period ends, you will start paying interest at the standard rate, which makes your camera much more expensive than you planned.