It's always a tough call to say a specific team that you could trade down with. No one knows what GM's are thinking, but every year trades are made.
In this case, the Bills could trade down pretty much anywhere below the last couple picks in round 1 and be able to pick up Mangold and an extra 2nd (at least).
With 2 second rounders it is likely they could pull in two of Jean-Gilles, Spencer, Joseph, or Latui, all of whom I believe can start this year.
They could also have the choice of picking up tackles like Winston, O'Callaghan, or Scott if they choose to go that route....though I believe the guards offer better value/impact.
Again, it sucks projecting a trade down scenario, but if they can get another DT to sign, I think it gives us the best bet to help the team.