That's epic i'm glad you found one with a lot of extras for a solid price, $2999 isn't too crazy for the 223 upper. They are kinda wonky long with how slim and long the m11 lower already is, but still full auto 223 would be nice and much cheaper than an m16 haha.
I'm looking at getting an m11 my self, the versatility with Lage is making them pretty nice rigs. I was thinking about an Uzi, but the fire rates kind Eh. I also was looking at an FNC with FA and 3 round burst, but then again for $8500 I could get a standard m11, 3k get the 223 upper and have a few calibers to choose from. I really like the K design on the m11, crazy nice cqc gun.
If you can find an Ar180, the 22lr ones with the 275 round drum. Those are sick, it's reported like 1600-2000rpm. Like a nest of Hornets, youtube has a video of a guy firing one at night with tracers, talk about a star wars light show. Its like a small scale a10 warthog haha.