For a midlength, in the price range you had alluded to, I would recommend BCM. Order a basic lower from G&R Tactical at $340 and the upper from G&R or BCM, which is complete sans a BUIS and should come in at around $550. If you could spring for a little more, the BCM cold hammer forged barrel is an option.
I, at one time did own a RRA midlength, back when they were first released and got rid of it after a couple of months. I did not have any problems with it, I just didn't care of the barrel profile or the fact that the M4 feed ramps were machined out of the receiver post anodizing.
I don't know what profile the Operator comes with, but I can tell you that the profile that came with my rifle (not sure if it is what RRA now refers to as medium weight or heavy weight) made the rifle a pig to shoot and carry. Also, as far as the barrel is concerned, I would take a look at the front sight base. If memory serves me correctly, the particular front sight base is secured using set screws. Many would disagree with me, but I prefer an exposed front sight base to be pinned to the barrel.
Finally, on my rifle, there was no staking on the gas key, which is easily remedied. Not sure if RRA has changed the manner in which they assemble the bolt carrier group, but it is something to be on the look out for.
Like I said earlier, I would encourage you to take a look at a BCM rifle. For about the same price, you will get a more basic set up, but it will be an outstanding rifle to start with. I say this because if experience is any indication, you will most likely end up experimenting with many of the components like the grip, stock, and handguards.