Hi FISD,
A couple of months ago I watched the feature on Netflix about the 501st Legion and discovered this amazing community of Star Wars fans and dedicated costumers. The best advice I've found at this early stage is to do loads of research so I've enjoyed many hours of poring over numerous posts, gleaning knowledge and building up an idea of which costume to start with and how to go about it.
Whilst I enjoyed the prequels and sequels as they were released, I grew up watching the OT so these episodes represent the classic Stormtrooper for me personally. For a newcomer such as myself there are a number of factors inclining me towards the ESB TK. As they're UK-based RS Propmasters seem a logical choice and have a good rep for screen authenticity.
I'm really looking forward to both the build phase and out trooping, joining the 'bad guys who do good'.