Yes, the same sources as everyone else is using.
The directors have themselves confirmed Bucky is returning as the Winter Soldier. .
No, the source I am using for the plot summary is Marvel Studios. They put it out with their casting call. Everything else is speculation. Yes, Sebastian Stan will be in the movie but everything else has been pulled out of someone’s rectum.
To be honest, the Winter Soldier focused plot would make an interesting globe-trotting thriller tonally in line with Cap 2. But…you actually don’t need Cap involved with that at all. Unless they were going to do a movie focused on Winter Soldie, it’s just not needed…
The Civil War plot makes an interesting through-line with the Marvel Cinematic Universe post-Avengers 1.
I think Bucky will take the place of the Punisher in some of the beats of the story. Whereas in the comics Cap begrudgingly accepted Punisher’s help and methods but haaaated it. I think Bucky will come to Cap’s side and be willing to do what Cap won’t (kill-kill–stab-stab!) and Cap will feel guilty about allowing it and using his friend that way.