I know this thread is basically dead but I'll throw my two cents in. I have been working through my own build and have tested cardstock versions of different parts and I can say the final armour is 70/20/10 but heavily modified. It is 70% Halo 3 armour and 20% FUD/Legends. The chest, codpiece, thighs, shins, forearms and hand plates are mostly Halo 3 versions with some minor simplifications and other additional details added over the top. The Shoulders and Boots are FUD/Legends versions with far less modifications. The pauldron (big shoulder pieces) is the most modified from its original, its shallower in depth and its narrower. It also has minor detailing added but its shape is still similar.
The last 10% is a heavily modified helmet. It has the silhouette of the Halo 3 helmet but with the back section of the Halo 4/5 helmet melted into it. This suit is a hodgepodge of armours and you can't pin it down to one suit to base it off.
I hope your build goes well. This suit is hard to figure out while the game is still teasers and trailers.