Putting together 6mm ARC AR, what brand upper/barrel?

[repoman1984]

I am looking for a new project, an AR platform long range rifle 500 yards or as far as a semi-auto AR-15 is capable of really . I am very familiar with AR's having built a few of them over the years including the years when you could build the lower from scratch. Anyway, I am looking for recommendations for the upper. Should I buy a complete upper? Should a I buy a naked upper receiver and install a high-end, name brand barrel? I'm looking a lot at 6mm ARC and plain old .223 wylde. I do not reload so I would be relying on good factory loads, I don't mind spending the money if I will get good accuracy for money spent. Does anyone have experience with good brands? Uppers?