After running the k4 proto and having ran a k3, i would say if you haven't ran a top tier backpack before get a used k3. Should be cheaper at the moment and you won't lose much if you sell it to upgrade next year.
Both a great.
The price is a little high because it's not 3 days until the hunt, he will get it sold, most likely at that price. If it was a shorter window it would probably be lower.