I was wondering from reading your post if your brother has a good knowledge of the dizzying array of parts and options on the market? If not, maybe he won't care. If he does, have you considered just ordering the basic parts (like a 16" barrel, I agree with Rob about the length, lower, LPK, and maybe some other essential function components) and then mapping out the build together? Could be a fun brother/brother project.

No offense to everyone, but I just wouldn't want someone else putting together my rifle. I think the difference in opinions just on this site should tell you that not every two people's "perfect" is alike. Maybe you put together this amazing rifle and your brother agrees with Rob that Vltor uppers are ugly. I'm a billet upper/lower guy and I have just never found them to be lacking. But some guys really don't like them and that's OK too. Maybe he shoots the rifle and you put a 12" rail on it, a tac light, forward grip, back up irons, maybe some rail covers and he thinks the gun is way too front heavy. Maybe you put a 9" rail on it and he would have really liked a 12" for a long sight radius. Maybe...maybe...

I just think that if you are going to honor your brother it should be with a rifle that reflects his tastes and opinions. If it was me, the fact that you took this initiative would be what was important to me. I just don't think you are going to create his ultimate weapon with input from others. I think it needs to come from the source. Since he is active duty, he might even have some strong opinions.