A good portable projector turns any blank wall into a movie theater. You want one that lasts several years, so focus on build quality and battery life instead of just the lowest price tag.
Wait until the fall when brands drop their newest gear. That is when last year’s top-tier models usually go on sale, giving you high-end tech for a fraction of the original cost.
The 18-month window is a deadline, not a gift. If you still have a balance once those 18 months pass, you will start paying interest on whatever remains, which makes the projector much more expensive than you planned.