The only thing you will be able to use for your 9mm build is your .223/5.56 lower (including the tube and the stock). Everything else you will need to replace to shoot 9mm rounds. You won't be able to use your A2 upper. You will need a new complete upper.
You will also need the magwell, as you have stated, and a different buffer. Generally people use a different hammer for 9mm, but there is some disagrement on this - depending on what type of hammer you start with. So, in total:
- complete upper
- magwell
- buffer
- hammer (probably)
Edit: If you plan on using an A2 upper, you are probably going to have to be an AR-15 armourer/machinist to get it working - but more likely you will bugger up whatever parts you have and not get a working gun. A .223 AR is gas operated and uses locking lugs like this:
The locking lugs of the bolt lock into that:
The gas tube has nothing to do with the 9mm principle of operation, so you will need to get rid that too.
On a 9mm AR, there is no bolt/bolt carrier group. It is all "one piece". There is no rotation and no locking. It is just a heavy "weight" that blows back like the slide on a pistol as each round is fired.
Save yourself a lot of time, money and frustration and just buy a complete upper from Questar, and the other parts you need.
This is a .223/5.56 bolt carrier group:
This is a 9mm upper with the one piece bolt/weight:
It's not like a .22LR conversion kit. You can't just swap them in and out of the same upper.
