First, please fill in the rest of you profile, especially your location, so the right people can give you the right advise.
Second, purchasing of parts on ASC requires the purchaser to be Age Verified as +18. Having you brother AV'd then buying for you is a good way to get banned for circumventing the AV process. The AV process in no way guarantees maturity, but at that age you are solely and legally responsible for your actions. You can find out about Age Verfication here.
Now to your questions. Tighbore barrels are not retricted parts so anyone can import them. They are fairly simple to install and are, IMO, the best upgrade for your dollar. Metal bodies are restricted, requiring special permits to legal import, so if you were to order one, it would most certainly get seized and destroyed by CBSA with no compensation. The foreend kit you posted the link to would probably destroy your plastic JG body (due to the weight of it), providing it's even compatable, but would fit onto a metal body easily and make the AEG pretty solid.
As for where to get them, as you're not AV'd, this site is out, but Google is your friend.