A new touchscreen makes an older car feel brand new, but a $600 upgrade is a chunk of change. The best time to buy is right after the holidays when retailers dump open-box units that customers returned after getting new gadgets as gifts.
You can often find these units at a discount if you check big tech store websites in January. Don't worry about the install cost; if you are handy with a wire harness, you can save the labor fee and keep your budget focused on the unit itself.
The 0% interest period is not free money, it is just a deadline. If you have a balance left when the 15 months end, you will start paying interest on whatever is still sitting there, so make sure your math stays on track.