I'm not sure why you are so dead set against putting one together. When you order a complete rifle kit, less lower receiver, all the parts are there and from the same manufacturer. I don't know about other companies, but PSA sells you the kit with an assembled and headspaced upper. All you have to do is put the parts in the lower, which takes 30-45 minuets and you have a complete gun.
PSA lower..................;..........................49.95
PSA M4 carbine complete kit less lower.....499.99
Shipping...............................................22.00
Rear sight.............................................35.00
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Total..................................................606.94
Because it comes from out of state, there is no sales tax. You save enough money to add all sorts of do-dads on it and still come out ahead. Also, in the case of PSA, the parts are all mil-spec and the barrels are made by FN (the US military contractor for AR's).
This is what you get when you are done: AR M4 carbine,
Or, if you prefer a more traditional look, you can build a AR15-A2. This is a rifle I built over 20 years ago and it still runs flawlessly,