You can't really compare Presidents across different historical periods. Reagan was a horrible post-WWII 20th century President. Buchanan was a horrible antebellum President.
Reagan is one of those horrible Presidents who we're lucky was President in the 1980s and not the 1860s or 1940s. He was a Nazi sympathizer and would have been pro-slavery.
You don't think Reagan would've waged war against a fascist genocidal movement taking over Europe? You're crazy. The dude invaded Grenada and funded the Contras in his term, two completely unnecessary actions. And the slavery thing is just too unknowable as its 120 years past his presidency.
I would even give you a pass if you said he would a McCarthyite, but I don't how anybody can take you seriously with comments like that.