First of all, welcome to the Cafe! Good to see a new face around here and hope you stick around
As for the trade, I don't like that deal at all. Ben to Flacco is a downgrade, Wallace to Fitz is a slight upgrade and McFadden to 1st pick is a huge downgrade. You have a tough roster to pare down because of the OP making you need/want to keep 2 QBs, but here's what I'd do:
Ben and Ryan (must keeps in your league format as QBs are the big scorers and you have two top-10 guys with upside), McCoy (no-brainer in PPR), McFadden (ditto) and then your favorite of MJD, Wallace and Austin in the last spot. Not sure of your starting requirements, but in OP leagues with 5 keepers, I'd say keeping 2 QBs and 3 RBs wouldn't be a terrible start.
You have two fringe WR1 guys, but 3 top-10 RBs, so it's a bit of a connundrum. If you could work a deal for MJD and a WR for an elite WR like Roddy or a younger, lower risk back I'd do that. With 7 easily keepable guys, you need to try your hardest to upgrade what you have before you lose that value to not being able ot keep them