Here are 2 suggestions for optics/scope:
1. Bushnell Banner 3-9x40 Scope with 6-inch eye relief.
bought mine for about $80 shipped from Amazon. Quality is excellent and the glass is very clear, all the way to maximum magnification. You can still use back-up iron sights even with the A-post front sight and low-profile rear sight. You just have to remove the scope, which takes like 15 seconds.
2. Burris Trophy 1x Red dot : $85shipped from Amazon but you will need to buy the extra riser to effectively use backup iron sights. If you want a non-magnification optic this is the way to go. Affordable and funtional. My roomate has it and you can Co-witness with your iron sights.
Of the two, the Bushnell is considerably longer because of the magnification obviously, but it really helps when you plan to shoot 100yards + frequently. I have the Bushnell and it is an excellent value and quality.
As far as the AR itself, check your local SuperWalmart if there's any in your area. I checked last weekend and mine has a Colt, Sig, Bushmaster, and DPMS in stock. I have owned a Bushmaster M4A3 and from my experience they are still decent quality for the money.
For reference here is the Bushnell 3-9 magnification scope. Beeotch is long but it does not interfere with your front sight since its a magnified optic.