Very long eye relief and magnification together are hard. The exit pupil on the scope is small with magnification, and that means your eye has to be in line with the eyepiece, and closer is faster in that case.
A red dot can be mounted in the scout position, no magnification, but is very fast. A typical scope is a 1-4x variable. Low end is typically Millett or Vortex Viper ($300-400), and mid/high end are SWFA, IOR, Meopta, Vortex Razor ($800-1000). Burris is in there also.
I have a 20" quad railed DPMS LR-308, and it runs great. However, the stock trigger is rough and I just replaced with a JP which is awesome.
Also - Armalite is the only maker of the "AR-10", and they have the lawyers to prove it.