I’m pretty open to any caliber which is why I didn’t include it, but now that I think about it I do have a lot of 5.56/.223 laying around so probably would be partial to that caliber.
Grab a lower and an upper and have what you want, my last pistol build cost me $350. I did snag the complete kit minus the lower, while it was on sale. Added a vortex optic for another $120.