The depiction of estate-owners and capitalists privately united with the Swedish Nazi Party is based on the real-life facts that the Swedish Nazis was mostly supported by, and led by, such figures in the country's history. Its membership consisted of a majority of upper-class citizens who loathed socialism, and even had connections with the Swedish Royal Family at that time, and the Army and Police in many municipalities.
One of the sources of inspiration came when director Hans Alfredson first heard Verdi's "Requiem". He instantly knew that he wanted to use it for a movie.
The local branch of the Swedish Nazi Party that's shown in the film is hinted and strongly suggested to be based on the real-life infamous Furugård Nazi movement in the 1930s in Sweden. Furugård, who was the party's so-called "führer", is never mentioned by name but it's generally understood in Sweden that it is his party that is depicted.
For authenticity, Stellan Skarsgård cut his own hair with a pair of sheep-shears, and used a mouthpiece to simulate the cleft palate that was the reason for the speech impediment that made others mistake Sven for being mentally challenged.