for the Early War, Great War Miniatures do a great 28mm range, (and we stock them here http://scarabminiatures.com/index.php?m ... cPath=5_25
There are other manufacturers, (Old Glory, Renegade for starters) but these are easy to get hold of.
Warhammer Great War is fine, we do gaming days based on that rule set, if you troll back through the threads you will find links to previous days we have run. We are just confirming a date for the next one. We do Early and Late war games.
Quite a few folks prefer Late War, as this gives more choice for troop types, and its not all trenches, a common misconception, but Early War is fine as well.
You will find on the Scarab Forum (http://scarabminiatures.com/forum/
) a few regulars very keen on the Great War in general, and 'encouraging' me to get on with our own rule set, so you can hop on over there to talk more about books and stuff, there are plenty to choose from, so you might want to select an army first.
Hope that helps, and welcome in to a fantsacinating period of history.