The bottom line is: you can convert pretty well any AR style receiver.
The kit (magwell, bolt, mags, and barrel) will be the extra expense. You can then have a complete 9mm AR carbine or sub-gun style shooter.
You just have to call around to the various retailers and price out the parts.
Usually buying a complete gun is the cheaper way to go, but not always necessarily.
Stay away from cheap crap. It's cheap for a reason and will eventually let you down. The price point difference between cheap and a relatively good quality isn't that much. Maybe a couple hundred dollars, and if that is a huge factor for anyone, maybe they should question the purchase.
9mm AR guns are a lot of fun, but probably won't let you compete properly in an AR/DCRA type of service rifle/carbine event. 9mm just won't do the distance.
You can use one lower, for 2 guns. Have a different upper for the 5.56, and one dedicated for the 9mm. Very easy 2 minute conversion done at any range with one allan wrench. Easy, peasy.
Don't buy crap, you will regret it.