Alright, so I sold my mage off my account back in 2014, but the method we used was xferring the character off my account onto his.

Using the "My relative passed and I would like to honor him in playing the toon etc."

So, I kept all my mounts and achievements that I gained on the account.

... and so, I still have achievements for the gold challenge modes, so when the prepatch hits tomorrow will I be able to get the token to buy the transmog set rewarded from the achievement for my new mage?

if you have the achievement, then i guess yes

I hope so, I guess we'll find out soon.