I had a quick look through William F. Buckley's published writings for references to Donald Trump. Got a couple of hits:
Most of the time it was when Buckley wanted to make reference to someone very rich but unseemly. For instance this, where Trump is the closest thing to the Democratic stereotype of the wealthy and powerful.
But more interestingly, in Cigar Aficianado (!) in 2000, Buckley fingered Trump as a narcissist and wannabe demagogue as the billionaire vied for the Reform Party nomination.
Buckley painted Trump as self-absorbed, shallow and lacking the tools and character for actual greatness. He swiped at the idea that businessmen offer much to the presidency. He doesn't predict Trump's nastier, truly demagogic turn, though.
It's impossible to say with certainty Buckley would have consistently opposed Trump. His magazine is anti-anti-Trump. Much of right-wing media has fallen in behind the president. Maybe he'd buy the argument that society needed to break out from a liberal paradigm...