I honestly think that most AR15s are just....AR15s. Getting a really nice once is a waste of money (often) IMO...and it's hard to find a "bad" one. It isn't a 1911 where attention to detail is needed. I'll probably get a lot of hate on this, but an AR15 is the ricer vehicle of the gun world. Most "mods" are pointless beyond their look.
That said, you can build your own if you have a barrel wrench, torque wrench, vice and a punch set. I helped someone build theirs, and I think we were done in an hour.