This looks fantastic so far. I was going to suggest pepakura but you're one step ahead of me :D
The most important advice I can give is to take your time with it. If it takes you three years to make a perfect suit of armour it's better than knocking out a cheap, shitty version in a week. The fact your using pepakura already suggests you will take your time but its worth mentioning anyway lol
Now there are several ways you can go about an undersuit. The easiest way would just be to wear a one piece black bodysuit, much in the same way stormtroopers do. But, for a more acurate look you could try to replicate the ribbed look that the inner elbows and knees of power amrour has.
Off the top of my head, the best way to do this would be to get a square piece of black, elastic like material, big enough to cover the gaps and also overlap into the armour.
Once you've got a piece that covers the gap with some overlap, you'll want to add velcro strips to the inside of the armour ( A note here, use the loop velcro, not the hook velcro for this part, as it wont snap on any clothes you are wearing beneath). Use industiral velcro as it will last longer and it's self adhesive is far superior.
Next you'll want to sew the hook velcro onto the material so it wont come loose. Once this stage is done, you'll have something that closes the gap and also joins the plates, keeping them more secure.
Now for detailing. Measure out the area of exposed material. Then take strips of leather, cut about 1cm wide and to length. Sew these to the material to give the exposeda area that ribbed look. It should look good, offer good flexability and also serve as a way to fasten your armour plates together.
Another tip I have is that the suit will get HOT!. This is a really important thing to take note of. If your out at an event all day with no way to cool down, it could end up being real nasty lol. There is an easy way out though :D
First, once the helmet build is finished, you may way to carefulyl cut out sections of the gril, to allow airflow into the helmet. You can cover the holes with black screen mesh from behind.
Next you want to get hold of some 12v computer fans to mount in the helmet. There are a couple of videos on youtube on how to mount fans in helmets that will help. I reccommend searching for costumebot for a really good one.
Once you have sorted the air con, it may be time to consider some sort of voice projection. There are countless ways of mounting speakers in the chest piece, linked to a mic in the helmet that will give your voice that voxx sound. It's best to experiment and see what suits you best.
I hope at least some of this helps, and if I think of anything else, I'll be sure to swoop in an mention it lol.