You want a telescope that keeps you looking up for the next decade, not one you outgrow by next summer. Invest in a quality optical tube and a sturdy mount now so you aren't stuck replacing cheap parts later.
Wait until right after the holidays to look for used gear. Enthusiasts often upgrade their kits in January, meaning you can find last year's top-tier models for a fraction of the retail price.
Remember that the 0% period is a deadline, not a pass to pay whenever you want. If you still have a balance left when the 18 months are up, you will start paying interest on whatever remains.