@Craig Dunkley is running Friday events in rF2 for the foreseeable future, I'm just helping out until things have settled down. I believe Craig will be running the Challengers Pack cars, but he may decide to run both packs at some point. Either way, I think you'd need to buy the Challengers pack to take part in Friday events, for the next few weeks, at least. If we take the Endurance Pack as an example, previous events involving new, paid DLC content, have been popular for many weeks, so I would guess that this will be the case with the Challengers Pack.
Sorry for the long post, but I wanted to be as clear as possible, because you need as much info as possible to come to your decision.