Re: AR build
White Oak Armament. Just go ahead and get a good one. These have better sights, and the front sight is mounted correctly. They also must have a free float forearm if you mean Service Rifle match. These will be 20" barrels.
But if you want an AR High Power "Match" rifle, you can also get that from WOA and it will have a 25"barrel of a different configuration.
First, get this book and avoide re-inventing the wheel:
The Comptetive AR15, by Glenn Zediker. Best 30.00 I ever spent.
Rock River, DPMS, Olympic Arms, Bushmaster...most of the majors make a pretty good CMP service rifle competition upper. You're gonna need a good trigger for that lower too.