I have done a bit of costuming (not leather, theatrical stuff) and I would do the bust in 5 parts, 3 front and you would probably get away with 2 on the back , also looking at the first picture it looks to me like its joined down the sides. If you read a bit more on breastplates there are some guys who are 'hinging' the armour a bit, a technique along those lines might make it more wearable. Or you might get away with a suede join down the sides for a bit more movement.
I guess with the molding the breastplate part.. it will depend on how large you are, if your a similar size then it should be ok. I would be doing a light foam layer then soft cloth if it was me wearing it to stop any rubbing issues. Or maybe di-sect an padded bra and stitch the cups into the lining.
Keep us posted, I am keen to see photos of how you get on.
and if you want real inspiration.. check out 'princes' armour.. his stuff is truly awe inspiring
Natalie