I built my AR15 9mm so that i could outfit it just how I wanted. I feel your pain as I thought the costs would be less but there are lots of little things that need to be altered. Not like just dropping a 9mm upper on it. As I don't have experince with their system you are looking into I will just give you some pointers I picked up during my handgun caliber carbine phase.
First, well they are just fun as hell. The long barrel makes them more quite and the muzzle hardly moves when shooting. I never have found that the AR15 had much recoil, but I was amazed when I started shooting 9mm carbines. I like mine for target practice and sometimes I shoot USPSA with it. The other thing I learded is that if I am going to end lugging a long gun I would rather it be a real long gun. Don't get me wrong I love them, but i would much rather have a pistol cambered in my rifle caliber, than a rifle chambered in my pistol caliber. So though I was thinking about using the 9mm carbine as an SHTF fan gun, it will instead be loaded to someone who needs a handy gun to keep themselves alive.
The other thing is that some people think that a 9mm carbine will be somehow better than a 9mm pistol in terms of power. Sorry I did many experiments with this. The problem is that because of the longer barrel most handgun loads are meant to have a burn rate that pushes the bullet for the leanth of a pistol barrel. In 16" carbine (unless you go the SBR route) the longer barrel acctually in some cases slows the bullet down because of barrel friction. Now if you load your own it would be a simple matter to work up a load with a burn rate for the longer barrel. You can also buy 9mm NATO spec ammo as it is loaded for much higher (its just above +P+ 9mm factory ammo) but it I think 8000 CUP lower than a proof load. this will get you better performance in the carbine.
I hope I did not dampen you spirits any because I really do love the little carbines, they are just not as great as it would first seem. Your idea of getting one that uses the glock mags is smart. I would not mind one of those myself. And As I am sure you know there is the Keltec system or a carbine conversion kit to make a 9mm pistol into a carbine. A buddy of mine has a keltec and it is a fun little gun. With my Ar15 9mm I can get head shoots at 100 without a problem, but any farther and things get very tricky. I put an Eotech on mine and with the low recoil and fast sight I can pepper a target with alot of rounds in a quicker time than with my AR15 .223. So Sorry if I got off the point or told you things you already knew, and have fun with you new toy.