You can learn about the different versions in this video from @GPLama
I ended up buying the “recertified” Kickr Core because it came with a full warranty. It also arrived in like-new condition. There was no sign that a cassette had ever been installed on it. Those are not always in stock but it’s worth checking. Most of the used ones I’ve seen appear to be overpriced and even more so if you value the warranty at more than zero.
Other bargains you could look for are the Jetblack Victory (supports virtual shifting, availability currently waitlisted in my region) and the Saris H3 (no virtual shifting but steep discounts currently).