Let's see.
You will need a barrel. I got mine from MAS Defense. It works fine, and was priced well. There are others out there. CMMG, I think. RRA and YHM all have good barrels.
Muzzle device of your choice. Doesn't matter, just make sure the threads match.
Are you going to run Iron sights? If so you'll need a gas block/front sight base. If mounting the sights on a rail, you can skip that.
Bolt. There are two kinds. Ramped and unramped. I recommend ramped, because it lets you use a standard LPK. The place I got mine from closed, which is a shame cause it's a nice bolt. But good makers abound. Again, CMMG, YHM, MAS Defense, I think PSA sells one now, JSE Surplus.
Buffer: A standard buffer lets a 9mm bolt go back farther than it needs to so it picks up some speed before it hits the bolt catch, This will lead to the bolt catch breaking over time. To combat this you can either get a spacer that goes behind the buffer spring, or buy an extended length buffer. The spacer is by far cheaper if you have a buffer from your lower kit. The 9mm Buffer is pricy, but heavy, and a one piece solution. I did a Spikes long 9mm Buffer, and have had no problems. 9mmar.com sells a bunch of buffers.
Magwell Block: Don't skimp here. Whether or not your 9mm AR is reliable depends a lot on what is holding the mags in place. There are essentially two types. Top loading, and bottom loading. (That refers to how the magwell blocks go in, not the mags.) Top loading are easier to index, less adjustable and harder to take out. Bottom loading blocks are harder to install, more adjustable, and easier to get out.
For top loading Hahn Precision is kinda the go to. For bottom loading, Hahn (bottom loading) or Spikes enhanced. Either one is going to set you back between $150 and $200. Don't buy the Pro-Mag or the C-Products.
Gas Deflector: You don't NEED a gas deflector, I run a slickside upper on mine with no door or deflector and am fine. But if you want one, you can throw one on. They usually come with the little half door that goes with them. I think they are as ugly as sin.
You'll need some mags. Make sure whichever mag block you get works with your mags. I like the Colt pattern.
Depending on which mag block you get, you might need a 9mm bolt catch.
Handguard/rail system of your choice.
That should do it.
Gratuitous pic: