Depends on how much money you want to spend and how much your time is worth. Personally I would go with a nice prebuilt. $1000-$1500 should net you a very decent gun. Accessories end up costing many people more than the gun itself. Focus on a nice scope and fore mount.
You could also buy a "bare bones" AR... If you want further customization without having to build the base yourself.
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The Cool Thing I bought finally made it to my dealer. It's an RPB M11A1. Colloquially referred to as a MAC-11. It's a full auto .380 with a ridiculous rate of fire. Thankfully, there are aftermarket slowfire uppers and such that make it a lot more shootable.