You are buying this setup to take notes, sketch, and get real work done for the next few years, so we want you to buy it once and get it right. Aim for the Air and Pencil combination that handles your actual workload instead of paying extra for features you will not touch.
Tech updates happen fast, but the current iPad Air stays capable for years. If a new version drops right before you buy, look at last year's model on sale since the specs are usually close enough to save you some cash.
Remember that the intro window is a deadline, not free money, and what happens if the balance is still there at the end is that you will get hit with back interest charged from day one at the standard rate.