A high-end indoor bike is an investment in your fitness that should last you for years of daily sweat sessions. If you choose a solid model now, you won't need to replace it when it breaks down in a few months.
Brands usually drop new models in early spring, which makes the post-new-year clearance events your best bet. You can grab last year's tech for a fraction of the cost while the retailer clears space for the latest shipments.
The 18-month window is a deadline, not free money. If you have a balance remaining after that period ends, you will start paying interest on the remaining amount, which can get expensive quickly.