Quick AR question(s)

I used to have an AR, but it was a 20" heavy barrel with a rifle length rail. Long story short, it heavy. Too heavy for me, so I got rid of it. Now i'm looking at getting another one to work on my marksmanship, since I plan on enlisting in the marines after I get out of school. My question is, Is there a good A4 style 20" rifle under $800? I'd like the opportunity to use the iron sights, but it would be nice to be able to put a scope/red dot on it later down the road. If there isn't, what kind of difference is there in the ballistics in 18 vs 20 inch? CMMG has the 18" govt. profile rifle in the bargain bin for $600, and I think it would work good enough for my needs. Any help would much appreciated, thanks.

CMMG 18" (bottom of the page)
http://cmmginc.secure-mall.com/shop/?cart=2230054&cat=161&