You can also make a case that establishment Republicans are on the same page as Clinton Democrats on foreign policy. Hillary Clinton's agenda for Syria (i.e. ground troops to overthrow Assad followed by nation-building) is identical to Marco Rubio and Jeb Bush's agenda. So the counter to saying that Buchanan (or, more to the point, Trump) "sounds like a liberal" is to point out that his opponents sound like another type of liberal.
The bottom line is that there have always been both liberal and conservative interventionists, as well as both liberal and conservative anti-interventionists. Calling your opponents "liberals" because they don't share your views is a game either side of the debate can play.
“You can also make a case that establishment Republicans are on the same page as Clinton Democrats on foreign policy.”
Absolutely, yes.
I posted a thread on just one such example about a month ago.
http://www.freerepublic.com/focus/news/3382823/posts
As far as the Iraq war, I do not think that was interventionist. It was self defense in response to a direct attack.