My advice to you is to post in the Colorado hometown forum and see if you can hook up with some of the members there and then check out their guns.
I am old school, I think you should start off with a "slick" AR15, no lights, optics, rails or kitchen sinks.
Build or buy, then make sure it works.
Then after playing with other AR15's that have the toys on them make your decision.
Buying parts vs buying a complete AR15 means you won't be paying the federal tax on a "complete" firearm.
Buying parts lets you get what you want.
I have a few mixmaster AR15's, I have not had any major fitment issues with the Oly, Rock River, Stag, Bushmaster, Colt, Fulton and Mega uppers/lowers.
You might have a tight takedown pin here and there, nothing that a little emery cloth on the pin can't fix.
That's my 2 cents....