I am looking for specific details on the Colt M4 "civilian" AR-15 that
will help me make the decision on what to buy and what to leave behind.

Only interested in a Colt, nothing else.

1). What is the best or most desirable LOWER to match the M4 UPPER?
I've seen SP1 lower, Gov't Carbine lower, R6721 Tactical Carbine lower,
R6601 Sporter Match Hbar lower and Colt Sporter lower.

I would guess that you would want something with a reinforced lower
and raised fence around mag well. Which lowers offer this config?

What is the best match

2). Which M4 UPPER is better or most desirable, the SMALL or LARGE Hole?

3). What do the TRUE M4's have, 1-7 or 1-9 twist?

4). How do I know if I get a REAL M4 upper, are there any markings, numbers
or stampings that designate M4? I've been told that the real uppers are the
Colt 6920 but I have also seen Colt R6721 AR15 marked as a M4.

5). What are the characteristics of the M4 upper that make it a M4 upper?

6). Are there any details or FAQ's around that detail the M4 rifle?

Any other suggestions and comments would be appreciated to help me make the $2500 choice.

Well, I can answer some of your questions.  The lower should be a small hole, reinforced, raised fence A2 model.  I believe these usually came on the blue labels on the prebans and mostly have the sear block and larger trigger/hammer pins.  Personally I dont have a problem with the block/pins as the benefits of the A2 lower outweigh the very slight nuisance of the block/pins.  Any of the models, including the Gov Carbine and the SP2, etc., could have these lowers so look for these.

The M4 upper is based on having lengthened feed ramps at the rear of the barrel.  These can been seen as having reliefs cut into the upper just below the feed ramps on the barrel extension, which differs from non-M4 uppers which dont have the relief cuts or as wide of ramps.  You definitely want the small hole upper, if using a small hole lower.  The M4 upper can be on any Colt AR with the removable carry handle.  It usually is marked "M4" or "4" or both over each other at the front upper part of the receiver just above the gas tube hole.  I have a R6721 LE model and it has the M4 upper.  Most M4 uppers are the removable carry handle type although the later Government Carbines with fixed handles are also M4 marked and have the special feed ramps.

The military M4's have the 1X7 twist but I see that the 16" M4 uppers Colt puts out now has the 1X9 twist, which is a better all around twist rate.  I believe the official Colt designation is R6920LE for this upper.

Do you have any suggestions on the TYPE of lower? H-Bar, Carbine, Tactical?

Do you know of any resources for the M4 on the internet? I've seen things spotty here and there but nothing like a complete break down. I do know the COLT M4 stocks should have a M4 buffer which is stamped H on the head.

Check out this site I found when looking, it seems to be pretty informative on the full auto - select fire M4.

I'm checking prices on gunsamerica.com and they range from $1800 - $3000. This makes me nervous as I want to get the best AND not get ripped off. Like the lower, if it's not a good match I may not know and pay top dollar for something that isn't what it's supposed to be. That's why I'm asking what lower to look for OR does it really matter. H-Bar would be heavy and reinforced but is it a good match?

[url]
http://www.specialoperations.com/Weapons/M4.html
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[url]
http://www.specialoperations.com/Weapons/Features/M4/Default.html
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Thanks for your input, please send more....