A high-quality electric bike should last you for thousands of miles if you maintain it well. You want to buy once, so look for a frame and motor system that you can actually get serviced locally rather than a budget brand that will end up as e-waste in two years.
Bike shops usually clear out last year's inventory in late autumn or early winter. You can often snag a better bike for the same price by choosing an older model that is functionally identical to the new release but lacks the latest paint job or minor software tweak.
That 0% window is a deadline, not a gift. If you still have a balance after the 18 months are up, you will be hit with interest on whatever remains, which gets expensive fast.