Well, for starters, you're going to need a lower parts kit, and an upper build kit with F/A parts, ejection port door/spring/rod/c-clip, gas block, charging handle, and some magazines. I would recommend 20rd mags for a bench gun. Oh yeah, you will also need a proper AR scope mount.
Barrel - I would recommend going with a different barrel for a bench/precision gun as well. If you are dead set on a Red X barrel, then use their 20" 1:8 twist barrel instead of any of their 1:9 twist 16" barrels. If you are wanting to shoot small groups you will likely wind up shooting longer projectiles, and that is when you need the faster twist rate such as 1:8 or 1:7. I would advise you to seriously consider a different barrel, as that is not the place to save money. Maybe check out the Rainier Arms Match barrels.