A good mountain bike is a tool that should last a decade if you buy right. You want something with modern geometry and components that won't break on the first rock garden. Buying cheap usually means buying twice, so we aim for a mid-tier build that handles everything you throw at it.
High-end bikes follow a predictable cycle. New models usually hit the floor in late summer, which is when shops start discounting the current stock to make room. If you can live with last year's paint job, you can save hundreds while still getting the same tech.
Keep in mind that 0% interest is a deadline, not a gift. If you still have a balance after those 15 months, the standard interest rate will apply to the remaining amount. Make sure your monthly payments are automated so you don't end up paying extra for your ride.