Getting better lights makes night driving much safer and usually looks sharper, too. Prices for full LED kits tend to dip when manufacturers clear out inventory before new model years, so keep an eye on sales cycles.
You are looking at about $400 for a quality set. Since this is a one-time upgrade rather than a recurring repair, you want to spread that cost out so it does not sting your monthly cash flow.
Remember that the 0% period is a deadline, not a gift. If you have a balance left when those 18 months are up, the bank will start charging you interest on what remains.