Bionicle was my childhood as well. I don't have any experience with or much advice to offer on making armor, other than to look for tutorials on making it on Google or YouTube. It seems like EVA foam would be the best option, and I'd use clips to attach each armor piece and wear a black one-piece suit underneath it. I really hope this turns out well, and can't wait to see any progress you make.
Also, I was looking at Iron Man armor tutorials this morning, and came across this very detailed one:
You might find it helpful.
Done: Captain Malcolm Reynolds, Eleventh Doctor, Captain Levi (SnK), Captain Jack Harkness, Vash the Stampede, The Kid (Bastion), Booker Dewitt, Arno Dorian, Jaime Lannister (S01-04), Connor Kenway, Mad Max, Shay Cormac, Jaime Lannister (S04-05), Otso Berg, Jacob Frye
In progress: Jacob Frye 2.0
Future/considering: Snake/Big Boss (MGS3), Spike Spiegel, Mako (LoK), Bucky Barnes (TFA)