Upgrading to a convection oven changes how your food cooks by circulating hot air for faster, more even results. You will notice the difference immediately when your roasts finish quicker and your cookies bake perfectly on every rack.
Aim to buy during major holiday weekends like Memorial Day, Labor Day, or Black Friday when appliance retailers compete for sales. Before you commit, get at least three quotes from local installers to ensure you aren't overpaying for the labor.
Remember that the 18-month window is a deadline, not free money. If you have a remaining balance after the 18 months are up, you will start paying interest at the standard rate, which can get expensive quickly.