You can get the power armour off of the bodies of any of the Brotherhood soldiers you stumble on.
Dont think I have found a national guard building yet. But from what I have been reading, and hearing from other people at work who have it, no two people will have exactly the same story all the way through (I believe there are 51 different endings).
Even the small things such as the GOAT test at the begining, will effect things you can and cant do. I am currently a small step away from being level 10, and have some shit hot weapons at my disposal.
