Personally, I'd recommend you begin with the helmet. It's the most iconic part of the costume and the one piece you probably want to get as perfect as possible. As an added bonus the helmet makes for an excellent display piece while you're still working on the rest of the costume. My helmet I keep out and displayed, and well out of the reach of little hands, and seeing it every day is what keeps me coming here and keeps me wanting to upgrade parts of my costume that I just don't think are good enough.
Btw....welcome to the board and to your new obsession! Good luck with the build!