A high-quality drone is an investment in your craft that should last you several years of field work. Buying the right model once saves you the headache of replacing budget gear that fails when you are miles into the backcountry.
Check the release cycles for major manufacturers before you buy. If a new version is coming out in a few weeks, last year's model will likely drop in price, giving you the same performance for less money.
That 0% window is a hard deadline, not a gift. If you still have a balance once the 18 months are up, you will start paying interest on whatever remains, which is the fastest way to turn a good deal into an expensive mistake.