Need Advice RE AR 15 Purchase

First post here. Looking forward to learning some things. Though I'm an "old hand" with many other types of firearms, my experience with ARs is limited to handling them. When I was discharged from the Air Force in 1966, I turned in my M1 Carbine at the armory while observing the first shipment of the new "Mattels" arriving at Wurtsmith AFB in their wooden crates.

Forty-two years later, I plan on buying an Ar15 for shooting in the Service Rifle Division (CMP) at Camp Perry. I would appreciate all advice, opinions, suggestions and inputs regarding the choice of said rifle configuration (at least to start with, as I would like to expand on this platform for other purposes later, including the use of a scope but without resorting to the purchase of a separate upper).

A friend recommended getting a Rock River NM A4. He thinks I should be able to buy one of these new for around $1,000.00 or less. What do you guys think? Thanks.

Edited to add : Initially, at least, I want to buy a complete rifle. Upgrades, downgrades and other modifications can maybe be done later.

my range buddy uses a Bushmaster DCM ''20 A2, a little more at $1100,while i am sure there are better choices, he does well with it and purchased it because it was not very expensive, as he didn't want to spend to much for a competition that he may not continue(just starting out). he doesn't say, but ill bet he really wants to qualify for DCM purchase.

A White Oak Armament upper mated to your choice of lower might work for you.

http://www.whiteoakarmament.com/complete_uppers.htm