I know Hegel greatly influenced Marx, but have any others been important in the philosophy of Marxism?
Feuerbach greatly influenced Marx since it was his philosophy that made him give up his Left Hegelian idealism for materialism. A lot of the philosophers he critiques, especially those that aren't his contemporaries, one could argue are influences.
Marx and Engels hated stirner with a passion, some early marx writing is dedicated to just ripping on stirner.
Aristotle, his ideas of republican catagories and science are all over Marx, as well as stuff like species being.
German idealism like hegel of course, though marx studied under one of hegels students.
Rousseau somewhat also. The utopian Socialists as well.
https://m.youtube.com/watch?v=0AwdihHME1M This podcast episode is good. You can probably skip the first 5 min. Episode 80 & 82 are good too.
Bakunin. You enemies have to influence you somehow, right?
The various different Utopian socialists, and the early anarchists come up a lot. Of course if you are looking for straight philosophy Aristotle is one, and it can't hurt to be familiar with the general milieu of German Idealism.