First, post count does equate to internet experience but not necessarily rifle exerience.
Alot if not most of the M16s in the military have been reworked and parts replaced to the point that they are in fact a kit rifle made of spares and often used ones. There are a number of members here on the site who work in this field.
Both the new factory rifles and kit rifles I both own and have fired for the most part are tighter than the military rifles I have fired.
I have taken great care in part selection and assembly of my kit rifle and quite frankly it is one of the finest shooters in all respects in my collection. Yes it has a Colt trigger assembly.
I recommend you not pay as much attention on the A1 vs A2 on your upper as to the barrel. Like you have said yourself, you don't intend to use the adjustable sight so after zeroed why should it matter which one you have unless you are going for some authentic look. This will open up your barrel selection options a tad. I personaly am a big fan of chrome lined barrels if for anything they are easier to clean and will out last, number of rounds, a non-chromed barrel. On the other hand, if driving a nail is your intent there is a slight advantage to the non-chrome barrel, properly cured of course. For the average plinker which there aren't many on ARFCOM, a non-chrome will last a lifetime or two. Once again I stress the chome lined is my preferance especially on short barrels due to cleaning and durability.
Complete kits are typically less expensive than buying separate component kits, however trigger assemblies, J&T has their Colt with all lower parts for $55, butt stocks, and uppers both with or without charging handles and botls are available so you can build the rifle you want with the features you want.
Most of the lowers as long as it's forged and machined to specification will function well. The real difference is typically in finish. DPMS is known for their teflon finish which is quite smooth and dark black. Matching that to an upper not DPMS may be an issue cosmetically. The only lower I've had fitting problems with has been an Oly but then that may have been a fluke since my experience was just one. I'm not familiar with the Grizzly. Personaly if at all possible, I would rather see the lower I am buying so that I may inspect it for the proper number of holes, burrs in the machining, and the finish. I would gladly pay the extra $20-$30 in order to get to do this.
Hope this helps.
Welcome to the site and sorry for the previous banter.
Tj