Hey Adam. Welcome to Cloud City Garrison. Determining which clone you want to do kind of depends on your budget availability of costume parts. Right now, I think the phase two is probably easier to come by than the others, just because those are the easier helmets to come across. But don't quote me on that. Take your time with your costume and pick the collective clone brains here and over at CTN (clone trooper detachment forum).
We have armor parties all the time - one today, in fact - so you won't ever have to take on the task of costume construction, alone. There are people here that will help you out!
Good luck with the build.
- Bill