I know that Completing the "complete all (B, C, D etc.) Challenges" challenges unlocks different costumes but those challenges arent showing up anymore? Do you only get one chance to unlock costumes?
In my experience, these metachallenges are static, and will remain until you successfully complete them.
For example, the Wolf has four metachallenges for rank D through A (in addition to the three regular, random challenges in each rank) that each unlock a part of the "Guard Dog" set. On my first playthrough I completed metachallenge D and B, netting me two pieces of the set. The next time I started with the wolf, the metachallenges for rank A and C where still there, which allowed me to complete the set.
Once you've unlocked an item, you can usually purchase additional copies of it through the in-game store.