You are buying a proper turntable and speaker setup to actually listen to music at home, not a cheap plastic all-in-one player that ruins your records. A good system should last you a decade or more if you pick the right gear from the start.
Audio gear has a predictable release cycle, meaning you can often save serious cash by waiting for last year's model to go on clearance right as newer versions hit the shelves during major gift-giving holidays.
Remember that the intro window is a deadline, not free money, so if any of that $800 balance is still there at the end of the 18 months, you will get hit with interest on the original amount.