Trump will never be allowed to be "authoritarian", Hillary will, with the aid of an adoring media.
Who would stop him? The Republicans (with few exceptions*) couldn't even be counted on to stand up to Obama, and they were supposed to be the opposition party.
With Trump, they would have even less of a spine because they are supposed to support the party.
Add to that, Trump's history of cronyism, he has decades of experience learning how to and practicing buying off politicians. He would have no hesitation just buying the support he needs.
One more factor, the people who would likely oppose authoritarian type actions of big government, ie, the Right, for the most part, have shed that desire. One of the mantras is they want their own dictator now. They want someone who plays just like Obama, with a pen and a phone, but thinking they are fighting for them. So even a large block of the so-called Conservative public wouldn't stand up if he acts that way, they would cheer him on with thunderous applause.
I wouldn't even count on the media to oppose him. He is the media's cash cow. Even though they feign opposition, they have admitted that he is good for business. Right from Les Moonves' mouth for example:
http://www.hollywoodreporter.com/news/leslie-moonves-donald-trump-may-871464 The thing that worries most of us though, even with all that being said, is Trump doesn't have the biggest opposition to acting like an authoritarian that would be necessary- his own core values.
*Updated with his addendum- even the Republicans who have a history of standing up to big government even when it isn't popular (like Ted Cruz and Tim Scott) have been all but destroyed by the Trump movement so those last bulwarks that have shown a spine have been rendered inept.
Time and time again, he has shown his own core values are as solid as quick sand and he'll change them to suit whatever purpose he needs. So he doesn't even have the core value of respecting the Constitutional nature of our Republic and limiting the power of government to keep his own conscience in check.
..and you wonder why so many of us worry about what he would do to our own side.