Can I pry a little? How tight is your budget? If its doable maybe spending a little more could get you a lot more. How much of a rush are you in?
Aimsurplus.com sells Spikes Tactical uppers and various lowers for a good price. I just checked and they are currently sold out of the Spikes uppers. You could call them monday and ask when they will have them back in stock. They are very nice people and regularly get shipments in.
You could also look into a Del-Ton assempled rifle. I think I priced one for around $650, though I dont remember the specs. Its not operator quality but I bet it would get any of use through a day at the range. You can also pass on their bolt and purchace a DD or BCM bolt to slip in.
For me ARs are a major purchase. They are expensive and Im poor
. Def. not a winning combonation for me. Ive learned to slow down and be patient or I usually miss out on a good deal. Theres an insane amount of great options out there. I doubt anyone here would hesitate to answer any questions you would have.
Lastly, give midlengths a look. You would probably like them a lot over a carbine gas system.
Edit: I just went and looked and the $650 was for a rifle kit not an assembled rifle. Their assembled rifle would be $750
You might want to look into a higher end used rifle. I usually use google or just browse WTS ads.