Adding an island changes your kitchen from a hallway into a room where you actually want to hang out. It gives you space for prep work, morning coffee, and extra storage that hides the clutter.
Aim for the winter months when contractors are looking for work. You will likely find more availability and room to negotiate your quote compared to the busy spring season.
Remember that the 0% intro APR is a deadline, not free money. If you have a balance left over after the 15-month window ends, you will start paying interest on the remaining amount at the standard rate.