The Pirates are going to be looking for a first baseman or a corner outfielder, with a bigger need being in the outfield. Jose Tabata and Alex Presley have not gotten the job done to this point and the Pirates don't look like legitimate contenders with guys like Josh Harrison, Garrett Jones, and Drew Sutton covering the corners. There is a possible internal solution in Starling Marte, but you don't want to put too many of your chips on him at this point in his career.
Ideally you would like a guy the Pirates could get by just providing some salary relief. We don't want to lose big time prospects like Cole, Taillon, Heredia, Marte, and I'm just not sure how much we could get with names like Rudy Owens and Jeff Locke. It's going to be interesting to see. The Pirates need a guy that can hit home runs, and a left-handed bat is preferred in PNC Park. Right now Chase Headley makes the most sense to me, it just depends on if they can get him without losing too much from the farm system.
Lots of baseball to be played before the deadline still, and things no doubt be changing consistently until then. Keep checking back for updates.
**edit** I may soon have to add Jed Lowrie to the list. The Pirates haven't been rumored to have interest in any shortstops, but you can certainly see that becoming a big need down the stretch. Lowrie doesn't have the best stick, but could certainly help the club out, and the Astros don't have him under contract after this year.