Any one of the three would do me fine.
Cahill is perhaps the "safest" option in a sense, besides the price, because he is athletic and mobile, young, English, a goalscorer, and has proven Premiership pedigree to a certain extent.
Dann, however, is less refined, but a fantastic ball winner who shows fantastic heart and grit in his performances. Add into the equation that he's now playing for his boyhood club and being coached by one of the finest coaches in Steve Clarke, I think he would be an excellent choice.
Zapata is one I don't know much about, but from what I've seen, he is good in the air and has decent pace as well.
Exciting times ahead