I know it sounds crazy, but unless you've got lots to spend on pre-made, pre-painted parts, one year is probably not going to be enough time. If you're lucky you'll catch a helmet for sale in The Cargo Hold, but if you're talking to one of the guys on here building em, there's usually a bit of a wait for the good ones. Of course, the helmet is the most important piece, but there's a lot more work to be done and it's rare to find someone selling a complete costume, let alone one that'll fit you.
I've got a really flexible work schedule, experience painting, and a garage that's mostly unused, so I thought 6 months would be plenty of time to build myself a fett. It's now a year and a half later and I'm still working on it. It was wearable in 6 months, although slightly embarrassing next to the other Fett's at Dragon*Con last year. It was almost a year before I thought it was worth posting pictures of the whole thing on here, and now I wanna delete those old posts because since May I've found better parts and built better parts (my skills seem to be evolving at the same rate as the costume). To me, it's become a neverending game of asking myself "Does my Fett really need that part?" or "This part doesn't look quite right, should I replace it?" followed by "Is it worth it to buy one, or can I build it from scratch and/or found parts?"
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As far as advice on which helmet to buy, I have only had my hands on a few, and they're not the most accurate available. I will tell you that Cold Cast is heavier than you think it will be. From what I've read, it's a mix of fiberglass and aluminum. My earcaps are cold cast, and they definitely feel like metal (although you can bend the thinner parts if you're not careful).