oh it look like some of the armor of minecraft mod (like the one add by ic² for example) ! really looking for that !
the way mechanizedIT is going seem to be a good one, for the accessory thing, i would look at how they did the attach for weapon and use that as a model to make it (like for the ammo of weapon too, making for example the helmet take a new glass that will improve night vision or one that will incorporate a zoom and so you need to choose the one you want 
and activating it with a right click)
really interesting gankey69 looking forward to see how you did it 