Elliot (Rami Malek)'s dealer 'Hard Andy' who deals drugs from a suitcase in a seedy hotel room is undoubtedly meant as a counterpart to 'Easy Andy' the man who sells Travis his guns from a similar suitcase/hotel room in Taxi Driver (1976).
When Elliot (Rami Malek) assembles his new rig and proceeds to check his email, the 3rd email is from Edward Snow, subject reads "FW: How surveillance affects democracy", with a tag "Leaks", is undoubtedly a nod to the accomplishments of Edward Snowden, the famous whistle-blower.
Elliot (Rami Malek) uses the login garyhost, this was the login used by Aaron Schwartz in his MIT hack.
Elliot (Rami Malek) and the ice cream man, played by Richard Masur, are listening to Orson Welles' radio broadcast of "War of the Worlds".