There is no rating for the Challenger, it is just like any other microlight. Any microlight instructor worth their salt will be able to teach you in it.
Without a license you will only be able to log hours as a student, so will have to fly with an Instructor and be signed out by one post solo until you gain your own license. Flying with another PPL will not allow you to log time. It might be worth you speaking to an Instructor or school before yo actually buy this thing.
As I said not a great aircraft, even closed cockpit it will be bloody freezing and I would not say it was a great machine for ab initio training.