I think I want to build a 9m/m AR. Gun will be used in training new shooters and for cheap CQB work on the pistol range. I want this gun to work EXACTLY like a .223 AR. I don't want different drills for what is essentially the same gun.

Which if any of the conversions use the stock magazine release, have a hold back etc....? The bolt hold back is not crucial but I definitely want to use the stock magazine release.

I want to use a Cav Arms lower, mainly for the weight savings and they are pretty inexpensive. What magazines will I have to use? I would assume that I will be using the Colt system? I have read several of the threads but it isn't all that clear to me for some reason.

Any help appreciated.

Will, I think if you make it much like the original Colt system, it'd come as close to possible to what you want to do.  You'll need (and I'll probably miss a part or two) a 9mm upper with bolt, the 9mm mag block of your choice, a bunch of Colt mags, and a 9mm bolt catch.  Once installed, all of the external functionality of the system is identical to a 5.56.  Of course you now have the grief of expensive mags.  Once you start trying to go cheaper, now you'll have issues with lack of bolt hold-open (such as an Uzi mag unless M60Joe converts the followers), unusual mag release such as some Sten conversions, etc.

Converting a 5.56 to 9mm, and then going back, is appealing because it saves $$, but I really think the best way to go is to create a true (and pretty much permanent) 9mm lower with a fixed mag block and Colt mags... not because it's "Colt", but because it's proven, and I think bolt hold-open is a good thing, especially if you are LE or .mil, and count on your weapon for your life, rather than breaking bottles or punching holes in junk cars.

Look to RRA for Colt-system parts.  Good luck!