I'm just debating how far I'll take this thing. It's a matter of need vs. want, but also a question of how useful it would be. At some point, it makes more sense to buy an AR.
As long as you don't think you can actually change the upper, put in an insert and have a 9mm AR quickly, cheaply and easily.
I tried that route, and it cost me dearly. I now have 2 AR's since I just plain bought another lower and stopped messing around.
The 9mm upper will cost you about $380, and the magazine adapter insert is another $150 from Spike. There are cheaper models ($50) but they are useless crap. So when you are done, you spent $550+ for a 9mm conversion kit, you still only have one complete rifle at a time, and you could have bought 2 complete 9mm carabines for the price of the conversion kit alone.
A 9mm carabine is not very powerful, averaging 1200 fps, so it is still a close in weapon that does not replace a 223 with 3000+ fps.
If you buy an AR in 223, and a 22LR kit for $150, you have a 'real' rifle and a way to shoot cheaply.