I tend to buy my uppers and build my lowers for a couple of reasons. First, I shoot left handed, so I want certain things to be ambidextrous - primarily the safety and charging handle. Second, I find the triggers on even some of the more expensive guns (DD, LMT) to be unacceptable so I'm going to swap in a Geissele. Third, I have a particular stock I prefer and tend to play with springs and buffer weights since I shoot suppressed. Fourth, even putting aside that I'm going to replace a bunch of parts on my lower, I find this approach saves me money.
Having said that, I have experience with several of the uppers on your list - KAC (although it's a 300 BO upper), LaRue, DD, and LMT. All are solid and reliable options, so your choice should ultimately come down to what you prioritize. If parts availability is an issue you might want to exclude KAC (and maybe LMT). If ability to swap out barrels and change calibers is a feature you like (and I do think it's worthwhile) it's tough to beat LMT. If accuracy is a top priority I've had good luck with LaRue (although I also second the poster who praised the somewhat outdated and out of favor Rock River Arms). I have a stainless National Match upper that shoots lights out. If you just want a solid hard use rifle that's also a solid value I'd go with Daniel Defense, BCM or Sons of Liberty Gun Works. Not sure why some posters are down on DD - they make rock solid (although heavy) rails and excellent barrels, although they can be over-gassed. Finally, I don't have any experience with Geissele uppers or rifles, but I agree that they have some great upgrades over a conventional AR and look to be a good value.
Finally, you might want to think about barrel length. If you build your own lower you can install a pistol brace rather than a stock ( I assume this is true in IL as well) and go with something shorter than a 16" barrel of 14.5" with a P&W. Personally, I am currently very high on 11.5" and 12.5" barrels as they are a good compromise between being short and maneuverable without giving up too much on velocity and dwell time. Of course, I shoot mostly suppressed which is another reason to like the shorter barrels and I assume suppressors are a no go in IL.
