Like i said, i just wasnt sure about it. The HW pre-req for that crafting quest is literally just being able to get into Ishgard(so expack registered and questline up to the right point) - the quest has you talk to someone at the crozier and you offer to help them more later.

Back on topic... really, if you know you want to continue the story and explore ishgard theres no reason at all to wait on the expack purchase. All you get early is the ability to make/fantasia an au ra character, but it doesnt add any cost to the subscription so if you know you'll get it later you dont have to wait at all. If you want to feel like you're getting something immediate for the purchase, then do it at about the time you get that first level 50 and what youre getting immediately is the increased level cap.