Hey Indy search through the AR forum, there are numerous times this has been asked. Bottom line is there are several options, the best of which is to build your own. WIth the lack of lowers and parts though, it may (IMO) be a better option now to buy an inexpensive complete rifle.
Quoted For Truth.
Search through the AR Part of the Forum. This has been covered and recovered and covered again for the sake of what's happening now.
A receiver is a receiver - If you plan on customizing it yourself with better parts down the line, getting any of the qualifying guns will be a fine choice. Don't get sucked into mortaging the house to have the name brand, underneath the shiny stamp on the side of the gun, they are all pretty much the same.
In all honesty, take any of the production guns, down to the part to part comparison and there is probably a 15% difference. The craftsmanship on the higher end guns is probably a little better, but only because they have a better finished product to install. Some guy who assembling guns at Olympic Arms could do the same job at RRA without any new "specialized" training.
JD