"Which piece of clothing are you going to buy?"
If you're simply asking "which clothes are you going to buy," then your response is correct. However, since the prompt specified "which piece of clothing" (although I think 'article' is better than 'piece') then you need the measure word 件.
It should be. My experience with native speakers is that they often use 要 where Duolingo has been demanding 会.
yeah it definitely should be. 要 = want/will (the intent of ’want’ denotes a future action)