You want a camera that lasts a few years so you aren't stuck replacing it every season. Aim for a high-definition model that handles low light well, as that is where most budget options fail.
Tech brands often push new gear in the fall. Look for last year's top-rated models during late summer sales when stores are clearing space for new inventory.
Remember that the 18-month window is a deadline, not free money. If you don't pay off the full $200 before the time is up, you will be hit with the interest charges on the remaining balance.