ts1313, if you're interested in a 9mm AR-15, Olympic Arms makes a 9mm upper that is designed to be used with any milspec lower - no special hammer or buffer needed. The Olympic 9mm system uses a Sten gun magazine with a block welded to it, allowing it to fit properly in the lower's mag well. There are a few manufacturers out there that make mag blocks so you can use a plain Sten magazine. Sten magazines can be found for $5-$10 a piece. The Olympic modded mags are a little pricey at $65 each.
Oly tends to get a bad rap from a lot of people, but these people never cite examples for their angst. I have not read a bad word about the Oly 9mm setups; eother withthe factory mags or aftermarket mag blocks. Everyone that has one seems to swear on their reliability with all types of ammo.
I thought about getting a 9mm upper for the same reasons you mentioned, mainly that it can be used at just about any indoor/outdoor range. The indoor range I visit every 1-2 weeks has an Olympic K9 complete 9mm carbine. They say it is one of their most-rented guns, and although they've had the gun for more than a year, they've only cleaned it 2-3 times. Depite the lack of decent maintenance, they claim the gun has never had a hiccup, and has been 100% reliable. I rented it once a few months ago, just out of curiosity. I now have an Olympic Arms 9mm upper on order.
FYI, so as not to make this post seem like a giant infomercial for Olympic Arms, I have read good things about RRA 9mm rifles, too. And Colt, if you can find and afford one. RRA seems to be a popular brand for the 9mm platform. Bushmaster also makes a 9mm. I haven't seen much on the Bushy, but I believe it is based on the Colt pattern, as is the RRA.
Good luck with your quest!