Well the sig barrels most of us used are gone. A upper and lower set from aero would be good. An aero lower parts set to match the upper and lower. Whatever brace you like.
I like the Armalite carbine recoil set.
Free float rail fore end of your choice.
Brownells, primary arms.faxon, balastic advantage. Joe Bobs,
Midway USA all good places to buy your parts.