So far as barrels, there's two really different ways you can go:
-Integrally suppressed (silencing baffles built right into the barrel)
-Threaded barrel (screw on/off a separate suppressor)
My understanding is that integrals can be quieter, but clearly aren't as versatile since you can't switch them around. Integrals also help with length, since they can be only 10" of rifled barrel and 6" of suppressor and still not be an SBR, since the suppressor is permanently part of the barrel.
There is an interesting compromise solution from a company called
Tactical Solutions:
That shroud is permanently attached, but the can can be taken in and out with the barrel still being "over 16" ", so no SBR legal issues.
Alternately, you can get a standard barrel and get it threaded, or add a quick-detach adaptor to the end of the barrel.