Good luck! We'll have a FF photoshoot during Kumoricon, so you should come by.
As for the Buster Sword, there are a lot of tutorials online for how to make it, as well as places you can buy replicas for relatively good prices. All about what you want.
But either way, good luck with it. Zack is amazing, but I can assure you you won't be the only Zack cosplayer at Kumoricon this year (if that is something you care about, being the only one to cosplay a character).
Those two colors represent two different classes in Solider. The black is first class where the blue is second class. If you want to cosplay Zack I would say make sure you get the hair done right, there are two different styles and if you want to cosplay the darker outfit then you need the hair style that he has in the first picture. The change of hair style is quite important in the game. I think that might be something people notice.
Yes, this is true. You have to be consistent with this, because crossing images can be a pretty big mess-up.