When you buy a tablet for school, you want one that will last through your entire degree. Aim for a model with a solid processor and good screen quality so you aren't looking for a replacement in a year.
Tech companies usually refresh their lineups on a set schedule. If you wait until a new model drops, the previous version often gets a price cut, which is the sweet spot for getting the best value.
Remember that the 18-month window is a deadline, not free money. If you have a remaining balance when the intro period ends, you will start paying interest on whatever is left, which defeats the purpose of the plan.