Your criteria for arguing against the public perception of "Buchanan as wacko" are not particularly convincing. For example, the term "well-respected" is extremely vague (respected by whom?).
As for success as author and commentator, there is no functional difference between Buchanan and Ralph Nader, except that Pat got significantly fewer votes than Ralph Nader.
Of the two men, Nader is equally "well-respected" (among a certain sector of society), and is historically much more effective. He, too, is (or was) a best-selling writer and well-paid commentator.
Nader and Buchanan also occupy the same basic niche with respect to their respective political parties and, interestingly, say a lot of the same things.
All of this is quite simply beside the fact that Buchanan really is regarded by the general public, and actively portrayed by the media, as a wacko.
Pat Buchanan is damaged goods. Whatever their actual merits, his ideas are tainted by the fact that they're his.