You dont need the Herbalist challenges.
See my latest video on it.
All other challenges must be completed in order to receive the Legend of the East outfit.
You can receive the outfit not sooner than the Ch3 mission "Horseflesh For Dinner"
This mission is needed to unlock the horse fence.

Also read the pinned comment for more information.

Also, the mission in this video is easy way to get the Gambler10 challenge.
Make sure after the poker game to finish the mission as usual.
so go get yourself the Legend of the East outfit.

UPDATE
Herbalist challenges are not needed, see my latest video on it.