Making a tube around a part of your body that doesn't bend makes no difference if you make the tube articulated and fancy. You still can't bend your upper arm or forearm. They bend at the joints, not in the middle.
Good luck with your build. Why wait? Start now. Seriously.
The final pieces cut out for the Iron Man suit. Forearm and elbow armour.
Once these are done, it's time to start modifying it all to wear!
http://i.imgur.com/wbwg5t8.jpg
It's not so much that it bends in the middle, but the length of the "tube" changes depending on the bend angle. Think about how much your bicep contracts from fully eccentric to fully concentric. Probably a good two inches. If you build that tolerance into the suit, then there's no need for articulation (though it's not technically articulation, I wish I could think of the word for "sliding sections"). I actually just Googled it and there's no such design in later suits, so I guess this was entirely a moot point. LOL
Cost of sheet metal, cost of robotics, cost of a decent welder... The usual.
Can't wait to see all this laid out!