A high-end induction cooktop is an investment in your outdoor setup. If you buy a cheap model, you will likely replace it within two seasons. A quality unit handles temperature control better and lasts much longer if you care for it.
Brands usually update their tech in late spring. Watch for holiday weekends in the summer when retailers clear out last year's models. You get the same performance for a lower price.
Remember that the 18-month window is a deadline, not a gift. If you have a remaining balance when the time is up, the card will charge you interest on the amount left over.