You are buying a portal to totally new games that should last you a few years of solid use, so you want to get the timing right on a setup that costs about $600. Getting into virtual reality is fun, but buying right at launch means paying top dollar for hardware that gets discounted once the initial rush dies down.
Wait for the software library to mature and look at last year's model if you want a deeper discount on the headset and starter bundle. That way you skip the day-one bugs and buy once instead of wishing you upgraded six months later.
Keep in mind that the 18 months is a hard deadline, not free money, and if you still have a balance left when it ends, you will get hit with interest on the whole original amount.