This is the easiest way I've found, I'm not saying its the best way!, just the easiest for me. There are a lot of 9mm AR platforms out there and I'm sure everybody likes theirs the best.
I built a complete AR in 5.56 and bought an OLYMPIC ARMS 9mm upper for it, (bottom rifle in pic). The OLY can use the SOCOM block & 32 rd surplus Sten mags. You can buy mags from OLY so you don't need the SOCOM block, but they are pricey. Also, It is made to use the standard AR-15 5.56 hammer and buffer spring.
This is a pop two pins, switch uppers and go set-up. I have many K of brass & steel cased 9mm thru mine with zero problems.
Another thing you might need is a little red wagon like you had as a kid to haul your 9mm ammo around. These are SUPER fun to shoot and love to eat ammo! A dedicated .22lr upper is great too, but thats a whole different discussion.
Good luck with the build, Ken