A new double-basin sink changes how you handle dishes every single day. The work usually takes a pro a few hours, but you should clear the cabinets and set aside half a day to be safe.
January is the best time to shop because home improvement stores often clear out inventory. Get at least three quotes from local plumbers before you buy to ensure you aren't overpaying for the installation.
The 0% intro APR is not free money; it is a deadline. If you have a remaining balance after the 15 months, the bank will charge you interest on whatever is left, which gets expensive fast.