You want a bike that handles trails for years, not one that falls apart after a season. Buying a quality frame and motor means you invest once instead of replacing cheap parts every few months.
Wait until October to hunt for closeout deals. That is when shops clear floor space for next year's inventory, and you can often snag a high-performance model for a fraction of its original price.
Remember that the 18-month window is a deadline, not a gift. If you still have a balance left when the time is up, the interest rate will jump, which makes the bike much more expensive than you planned.