I'm also a socialist and would like to share my opinions on this matter.
Lincoln becoming a Marxist is not too unrealistic in actuality. In the beginning of the book(which I'm still reading) he makes a speech condemning southern slavery and the greed of the British/French capitalists who profit off of it.
Lincoln is clearly disgusted by this notion that a "free" Britain and France are profiting from a supposedly outlawed institution.
Now, if we tack on twenty years to Lincoln's lifespan, he could very well become a Marxist with such a clear-cut mindset against capitalist profit making.
Marx was also something of a loathed celebrity of his time; As an educated man, Lincoln could very well have immersed himself in Marx's writings sometime before 1881.
As for my thoughts on the "tainting" of socialism by the former USSR, I'd have to
disagree that socialism in the USSR was as a totality, a bad thing.
Lenin's last letters, tragic as they were, pointed to his helplessness and frustrations with preventing the rise of Stalinist government. Stalin had planted spies within Lenin's midst, whom intercepted his secret letters that would have been used to condemn Stalin and the next party meeting.
Interestingly enough, Lenin's very last letter pointed to Lenin's willingness to provide total autonomy to minority nation states in the USSR. Specifically he announced that he would champion the cause of Georgian independence/total autonomy in the USSR. Stalin on the other hand was notorious for blocking all attempts by Georgian Bolsheviks to make Georgia as independent as physically possible as a federated socialist republic.
What does this all mean? It means that Lenin and his vision for Russia was betrayed.
When the last of the Lenin statues were knocked down in 1991, people blamed Lenin for their subservience to Russia.
In reality, Lenin wrote in one of his letters that the exploitative nation(Russia) should have
less powers then the exploited nations(the minority nations that were part of the newly formed USSR), so as to maximize independence/autonomy.
He even went as far as to say that he planned for the USSR's minority nations to have a right to break away from Russia whenever and wherever they pleased.
my two cents. 