I'm having legitimate difficulty figuring out how an intelligent person can consider Donald Trump a suitable candidate for presidency. I can somewhat imagine how someone might hate Hillary Clinton or the Democrats so much that they consider Trump the lesser of two evils (though I think that's debatable). I can also imagine single-issue voters holding their nose and voting for Trump because only the Republicans will support their views (Pro-life voters come to mind).
But to claim that he is actually a suitable candidate? What is that based on? He has demonstrated a considerable lack of knowledge, or even interest in knowledge. His speeches, at least when not reading them directly, are essentially white noise and bumper stickers. And when he goes off script and says what he really thinks, his narcissism is on full display. He is incredibly thin-skinned and vindictive, threatening to sue newspapers who criticize him. He comes off as a bully, attacking people on twitter even when it is strategically unwise to do so, demonstrating a complete lack of restraint. He boasts about being able to use power to get away with things, like assaulting women or not paying taxes. He refuses to release his tax returns and his excuses are demonstrably false. His record of making factually incorrect statements is ridiculously high, possibly only surpassed by his ability to say things that are incredibly unsettling and destabilizing, like encouraging foreign interference in an election, or musing about backing out of NATO, or asking why the US shouldn't use nukes, or suggesting he might not accept the result of a democratic election if he loses.
What exactly is it that makes him a suitable candidate?
I'll put it this way, being extremely charitable to Mr. Trump. Maybe playing Devil's advocate, you could say.
1. Someone clearly opposed by his own political party's establishment is an extremely attractive proposition. These voters want to see blood and change in their own party. They look at the party leadership and realize that they are like cattle for the elites. Evangelicals/conservatives in particular believe they are being exploited for cheap votes, while their social issues are being quietly abandoned (especially gay marriage, which centrist Republicans won't touch anymore, a big change from 10 years ago). That said, Trump is not the perfect candidate for this demographic, as he alluded to in his RNC nomination acceptance speech, and many of those voters preferred Ted Cruz. But by contrast, Hillary Clinton is unacceptable. Suitable as the lesser of two evils, yet again.
Other portions of Trump's base want to see blood in the water in Washington, D.C. They want to see a purge of corrupt and overpaid officials (and there are many, no doubt). They look at Trump with his partially self-financed campaign (seen as a major positive, since he "can't be bought") and his brash off-the-cuff statements, and they see a guy who could actually change things. A lot of nominees have claimed to "change the culture of Washington", but none have been so bitterly opposed by their own party. The implication is that a lot of politicians are scared of a Trump presidency, because they might not be able to survive it, and both their actions and words speak volumes.
(This is partly why Melan put Donald Trump and Bernie Sanders in the same sentence. They are two very different candidates, but they are linked by being opposed by their party establishments, as well as opposing certain things the elites want, like the Trans-Pacific Partnership.)
Thus, in conclusion, Donald Trump's suitability as a candidate is derived from his unsuitability. He is unpredictable, yet unprecedentedly hostile towards his own party's establishment. That being said, there is reason to believe that Trump could win and throw his supporters under the bus. He had the "Art of the Deal" propaganda book written about him to establish his credentials as someone who can COMPROMISE, and compromise could mean complacency towards misdeeds, or even his own participation in graft. It's not like the Republicans in Congress are going to help Trump conduct a witch hunt. Wouldn't it be easier for President Trump to sit back and do nothing?
2. "Lack of knowledge"
Not an issue. He can claim that picking top-notch advisers is what is actually important, and that this "skill" of surrounding himself with knowledgeable people is what made him successful.
We can see that he has already assembled people in advance to advise him, including well-known Republicans (going against his anti-establishment cred) and some more fringe choices. He has also published potential Supreme Court picks in advance to allay his party's concerns.
The lack of knowledge displayed in his speeches and in the debates can be chalked down somewhat to being a terrible orator. Remember that heavy breathing in the debates? That's evidence that he is not prepared for that format. Yes, I'm using his lack of experience to defend his perceived lack of knowledge.
3. "Narcissistic, thin-skinned bully" and the rest of the post:
This is a bad trait for himself and any potential President. But it doesn't disqualify him.
As long as he hasn't done anything illegal (that can be proven beyond a doubt) then he is OK. The taxes thing was a Presidential tradition, not a requirement, although his explanation for not doing so was BS. The statements on women were played off as "locker room talk" and the allegations that came out later were given the Cosby treatment: desperate women coming out of the woodwork to gain publicity with false or unsubstantiated claims.
Foreign interference in the election is a huge can of worms because it goes far beyond the statement you referenced, which he played off as a joke: "Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing. I think you will probably be rewarded mightily by our press." I think that is a reasonable explanation of the comment. There have been claims since that Trump has some sort of direct or indirect contact with Russia, as well as some of his campaign staff. There are a lot of scare tactics mixed in there. At worst, I think we'll find that Putin took it upon himself to direct assets to cause mayhem in the U.S. elections. Undermining the legitimacy of the U.S. elections is great for making Western liberal democracy look chaotic and unappealing. Helping one of the most Russian-friendly candidates get into office is just a bonus. Note that China has used Trump as an example of why democracy is dysfunctional, and might have as much incentive as Russia to do the hacking. Whomever hacked what, I think we can agree that WikiLeaks was handed some valuable information that will prove beneficial to the public and historians, and that the DNC's complete disdain of Bernie Sanders has been exposed. That's why I say that if Clinton loses this election, the Democratic party is going to have a little civil war because the leftists and centrists of (and controlling) the party are itching for a fight to redirect the future of the party.
You can find plenty of people who support the statements on NATO and nukes. Look at the little guy Trump supporter. Does he want to hear that the U.S. could be obligated to go to war over some European or semi-European countries he doesn't care about? No. What does resonate with him is the idea of member states not paying their fair share. He also agrees with Trump's isolationism, and sees NATO is just another instrument for the U.S. to act as the world's policeman. And let's say we do pull out of NATO. Does Russia then ride in and take all of Europe? That seems unlikely. As for the nukes, the Non-Proliferation Treaty is a dated artifact along the same lines, and is flaunted at will. India, Pakistan, and Israel all have nukes. We regard nuclear capability as a "red line" for Iran while allowing Israel to pretend like it doesn't have hundreds of them. I believe the actual comment you're referring to was about proliferation, specifically about whether South Korea and Japan should have the ability to produce nukes. The U.S. has a policy of not allowing Japan to have nuclear weapons. Japan has gone along with that but could amend their Constitution to allow themselves a real military and the like. It is also said that Japan could rapidly develop a nuclear weapon if it chose to. It comes back to the U.S. being the world's policeman and taking on everyone's problems. Japan and South Korea are U.S. allies that are meant to counteract China's sphere of influence. Would it be so bad to make Japan pay for their own defense and host their own nuclear weapons capability, especially if they chose this route themselves (to expel unpopular U.S. bases)? Would it cause a nuclear war with China? Unlikely. North Korea is a more uncertain problem, but they already have nuclear weapons that could hit at least South Korea. We already have MAD deterrents against catastrophic action by NK, although we are entirely ineffective stop them from downing a few ships or shelling a few islands.
About the refusal to accept democracy bit. "Accept the result with a phone call" style democracy died back in November 2000. The losing candidate will pursue as many recounts as fervently as possible. This may have been the ultimate politically incorrect comment by Trump, but it reflects the reality that the stakes are too high to back down. Part of Trump's argument was also that he can't accept a result in advance because their might be election irregularities. This is a pretty well debunked argument, but fraud can occur and I was amazed at how easy it was for me to get my voting card. Every bit of information required for me to vote has either been stolen from me in the past, or can probably be found in some hacked database somewhere.
At the end of the day, "suitable" really means "in reasonably good health" (and he may not be) and meeting the requirements outlined in the Constitution. A 35+ year old natural-born citizen that has been a resident for at least 14 years.
Disclaimer: I don't necessarily believe everything written above. Maybe it was all a joke.