Too bad that I have missed seeing this thread for so long a time. There have not been that many people that have made this costume, but I am one of them, so I could give you some advice. My description on Rebel Legion's web site is not up any more, so I will have to post a new one here instead:
Helmet: My helmet was made from a ABS kit from RA, but the swoop was rather small so I upgraded it with one from TK560.
Jacket: Here is a picture of the original.
I think that it looks like all seams were made in the same brown thread. Practically all seams are double-stitched.
I made my jacket. I took a pattern for a suit, transferred it to pattern paper and simplified it a lot. I used lightweight beige cotton twill. The shoulder pieces were cut from the same fabric and dyed. The lining was made from the same pattern as the outer layer but in another fabric. The collar was just a rectangle. I had shoulder pads and a zipper. In all it was not difficult to make but the pockets took a bit of effort. I can recommend that you add a lining -- it is very comfy.
I know others have used US Army BDU jackets and modified them, but I don't think they look as good.
Shirt: Modified brown shirt. I just followed a RFT tutorial.
Pants: The same you planned. Khaki pants and a black ribbon. Beware the pockets. It may help if you rip the inseam before adding the stripe and remake it afterwards, otherwise you will have to thread each pant leg onto the sewing machine which can be difficult.
Badge: Piece of sheet aluminium. Pin. Button covers "EAO series 01" -- the same series that most Imp badges are made of, just the circular type, glued on with epoxy.