Alright, Goon. Here it goes: RJrivero gave a great start, olympic arms is one way to go but I couldn't even find one available and wanted something with a last round hold open. Plus they do seem to be pricier and as RJ said proprietary parts.
The following deals with a standard AR-15 lower that you want to make interchangeable. If you go with a dedicated lower then pick a glock compatible lower and buy block mags, Done and done.
MAGWELL BLOCK
You will need a complete AR-15 lower with all parts installed, then you need a magwell block or magwell adapter this allows certain magazines to fit into the magwell of your gun and lock in place. There are a number of good options include a Rock River Arms (I have this one), Spikes tactical and Hahn precision are the top of the line. I'd of preferred spikes but couldn't find it at the time. I believe all three of these have a last round bolt hold open feature if the magazine you are using also has the feature. Normally, you will adjust the height of the magwell block a little bit up and down if your having any feeding or ejection issues, the spikes tactical gives the most adjustment but for the most part they should all be plug and play. Most have a screw on the bottom that you tighten to lock it into the magwell tightly. This allows you to use your mag release button without pulling the magwell block out too.
~ There are two types of magwell adaptor, one's that go into the magwell from the bottom where you put your magazine in and ones that go from the top of the lower but require removal of the bolt release mechanism to install. The last option is generally used on a dedicated lower that you do not plan on changing back and forth.
MAGAZINES
Next you need magazines that fit that magwell block, The best are Colt 9mm ar style mags like these:
http://www.brownells.com/magazines/...15-m16-9mm-colt-style-magazine-prod38850.aspx
There are many makers of these style of magazines but I've found that metalform works the best for me. Real genuine colt ar 9mm mags are very expensive. Promag versions are cheap and that's about it, they are cheap. I tried one and it is too tight for my block and I've heard of the plastic versions splitting at the seam when full. The advantage of these are that they feature a last round hold open (which I wanted, no exceptions).
~It is also possible to use a Modified Uzi magazine which simply means that the hole to lock the magazine in place in the magwell has been redrilled in the magazine body to be higher up but it will NOT hold the bolt back even on a magwell block that has that feature.
UPPER
Pretty straight forward really, you pick an upper you like and go with it. But be aware that the bolt it includes will make a difference, read the next section for explanation. Personally, I have and like my CMMG 9mm upper. YHM, RRA, CMMG, and Spikes Tactical all make uppers. I'm sure there are others I've forgotten, so maybe others will chime in there. Possibly worth noting, my CMMG upper uses a 1/2 x 36 threaded barrel, as I want to buy a suppressor it is a little odd and inconvenient so you may pay attention to that.
BOLT
There are two types of bolts when building a 9mm AR, one that is ramped and one that is not. The bottom of the bolt nearest the portion where the hammer hits the bolt has a portion of about 1 inch cut out of the slide making it a ramp rather than flat the whole way. The ramping is to allow for more travel of the bolt before the hammer is reset as opposed to a non-ramped bolt that slams the hammer back too fast leading to problems. A bolt that is ramped will allow you to use your standard AR or M16 hammer. To my knowledge RRA does NOT use a ramped bolt but recommends a 9mm hammer or pay to have the bolt ramped.
It looks like this:
I'm under the impression that you cannot use this hammer on a 223 so I avoided the whole problem and bought a CMMG ramped bolt and kept my original hammer. This is the way the Colt 9mm AR was designed as opposed to the RRA 9mm AR which uses the non-ramped bolt and special hammer. As I stated I went with the Colt style.
BUFFER
This one I haven't figured out completely, I bought a Spikes tactical combo buffer for my AR, This:
http://www.spikestactical.com/new/ss/lower-parts-spikes-st9bs-stt2-combo-9mm-p-235.html
Your supposed to use the actual buffer like any other buffer and it can safely be used on 9mm and 223, the extra little part is the buffer spacer and is supposed to go on the other end of the buffer spring to add a little tension to the spring to keep the bolt closed a little longer. I have done both ways, with and without the extra piece when shooting 9mm and I have noticed NO difference at all, I do like the spikes buffer though, very smooth in the 223 configuration. So it's up to you, I can't see any harm in using your stock buffer at this point.
To Put It All Together, in it's simplest configuration:
9mm upper of your choice with a ramped bolt, Magwell block and compatible magazine, Done! Go shoot and have fun!
The last thing I can think of is a website that you could use to buy some of your equipment. I bought a magazine from them and they were very good and fast shipping.
http://stores.9mmar.com/StoreFront.bok
Good Luck, Hopefully I cleared up things a bit!
Thats why 9mm uppers aren't more popular, why buy an upper when you can get a good complete carbine/rifle for the same price?
