I really see Buchholz as a major holdup here. I think he's probably on the same level as Lester in terms of willingness to trade (ie he is not going anywhere). For every big name that has come up in the last three years, Buchholz has been the obvious target for any team and Theo has never budged. Definitely don't see him budging after the dominance Clay had to end last season when he really started putting it all together.
At the same time, while Kelly and Westmoreland are both top 25 prospects in baseball, the fact that neither has passed A+ will probably limit their abilities to be a centerpiece for Gonzalez at this point. I could really see both players value skyrocketing with success in the upper levels this season, so perhaps a mid-season trade is more realistic.