Though the smart money is on A.J. and Bush, Ervin has a little more upswing potential. Ervin is in that magical 3 year and has better numbers than anybody his age with a similar number of career starts (i'm talking Bedard, Cain, )
Both A.J. and Bush are potential 12 - 14 game winners who'll get you a ton of K's and ERA's under 4. I'd be surprised if Ervin's was under 4.25 this year.
So, take one of the surer things and then snatch up Ervin for the up-side.
As for closers, you have 4 that are better than all that are available.