Most of the Marx's ideas like public schools, public healtcare, public voting with no property cenzus, rights for women, childs are done in modern world. Also things like international organizations occur.
Marx's teaching ? He predicted that rich class will get control over everything and that's... so true, 70% of world's wealth is in hands of few, and things escalated.

I am going to go for a more succinct claim: That someone didn't understand one iota of what he was saying and now his reputation has been tainted forever...

For one, The Communist Manifesto reads jack-squat regarding the existence of a secret police...

As many scholars, some of his work is valuable to this day, some was superseded by newer research. Overall there is a reason why Marx is still reading material for economy and political philosophy - enough of his work can be applied to this day.

Exactly. We all have no idea what Marx would've said about Stalin and Mao, but these basically perverted his intentions. If you have a vision for a better world that doesn't mean to murder everyone not sharing this vision. And after all I think Stalin and Mao didn't even pursued this visions, they merely used these to gain followers and use them to get powerful.

This. Marx believed in industrial revolution, what should make us to the next level, claimed that the work is the only source of wealth. 

You will kill me with laught, but it was USA who was first to introduce Marx's ideas - voting rights for women in Wyoming,  over 100 years before France, 10hours work day in Florida ( or Boston ? )

The worst thing is that Uncle Joe patented what would emerge as the "commie" modus operandi by not allowing dissenting opinions. The thing is that this wasn't a very leftist thing to do, but it was a very Russian thing to do. Russia has historically relied on strong leaders with a powerful secret police and much of the stability of Russian officials is endorsed by a brutal counterintelligence lobby. In other words, Russia has a deep state identical to the US, but far less transparent. Stalin inherited this powers and used them to enact communism his way, with a complete disregard for everyone else, and inspiring everyone to do the same. 

There's a reason Russia even as a democracy is very censorship heavy. This has never been a communist thing. This, since the ephemeral rule of Cathy Two's husband, has been an objectively Russian thing. The repression of communism is a product of Stalin's brand of communism Russifying the ideology and then everyone doing what they were doing since they were the only communist Superpower they could use as guidance. 

Same goes for Yugoslavia and Albania, but worse... 

If you go to an Slavic country, you should know anything originating East of the Rhine is going to have very vicious undertones. The Slavs didn't really go through Enlightment and Poland, Russia and the Ottoman Empire acted as an stopgap for any libertarian movement headed East. 

Liberalism and all the freedoms we use and sometimes abuse are a FRANCO-BRITISH export. Even Spain, Italy and the rest of Europe preferred their strongmen. The only reason they fell was because the world was run by France and Britain until the forties, then the USSR ran it until the early eighties, and finally, the US took over since then. This is also why the US saw nothing wrong with Banana Republics. Being repressive was in vogue back then. 

TL;DR Communism isn't repressive. Communism that relies on anything related, even indirectly, to the USSR, is, by virtue of being a Russian product. Being Russian is being repressive. That's how Russian society has run since the XVII century. 

Did Marx ever claim any of these things? He advocated for a 'dictatorship of the proletariat' as a necessary result of history in his view of historical materialism.

He did. He advocated democratic system, what is generally dictatorship of workers.

Do you have any quotes from him regarding that?

Sure, for example public schools is 10th point of Proletariat/Communist part of Manifesto.

Also, Marx agreed that there are countries like USA where workers can achieve their goals legally.