I'm right handed and cross dominant so shoot a pistol right handed but bring it left for my left eye. With a rifle that's not possible so my right eye does the job, no muss no fuss. Cross dominance bothers some people and others hardly at all. I wouldn't worry about it unless it really drives you crazy.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek