You want a kit that won't quit halfway through a trip. Buying a decent one once is better than buying a cheap one twice when it fails in the sun.
Tech updates happen every year, but solar panels don't change that fast. Look for last year's model during early summer sales to get the same power for less.
Just remember that the 18-month window is a deadline, not free money. If you haven't paid the balance in full by then, you'll start paying interest on whatever is left.