Honestly I don't think Dunn will turn it around and the constant strikeouts are so depressing. Hanley has slowly started turning things around possibly. Zimmerman is going slowly too. It's definitely a risk. However, there's a lot of potential with HanRam/Zimm. I would do this deal.
With you have Texeira at first you can definitely risk the loss of Dunn if he ever turns things around. You have Longoria at 3B and Rollins at SS, so I'm kind of wondering how you make this work for you. Also that leaves you with 2OF's........ Are you going to be picking up an OF?
I too am concerned about the positions you are receiving, but I think when you have the chance to potentially take back the two best players in the deal, you have to do it. Then you can worry about dealing your excess later.
You are definitely getting the best end of this deal. You will just have some situations with who to start, so if you are planning on making another trade after you get this one done I would understand. But if you can make him substitute Zimmerman out for a top-tier OF I would think the deal would be better for you.