Buchanan would never be nominated, and will never pass the Senate. He's crossed the line with the Bush family, and is way too easy for the Dems to charicature.
I don't think Bush is all that fond of Tancredo, so I don't see him getting the nod. Someone else suggested Rick Santorum, but that's too dicey with the Senate up for grabs. We need those guys in Congress anyway.
Among her other assets, Malkin is the daughter of legal immigrants, and would be tough to bork as some angry, white male fringer.