A treated room changes everything about your recordings. You will spend a weekend mounting panels to stop echoes and tighten up your audio. Your mixes will finally translate to other speakers because you aren't fighting the room anymore.
Wait for the monthly sales events online to pick up a kit. Prices drop when retailers clear inventory or push holiday specials, so keep an eye on your favorite gear sites before you pull the trigger.
That 0% window is a deadline, not a gift. If you have any balance left after the 18 months are up, the bank will charge you the standard interest rate on what you still owe.