The Harkonnen cannon Walter equips Seras Victoria with was named for the evil Baron Harkonnen, from Frank Herbert's novel 'Dune'. This is evidenced in the manga, as the "Spirit of the Harkonnen" appears in a dream sequence as, indeed, the villain from the David Lynch movie, Dune (1984).
The character of Paladin Alexander Anderson is based on actor Edward Van Sloan who played Prof. Abraham Van Helsing in Dracula (1931).
For those who never figured it out, the name "Alucard" is the name "Dracula" spelled backwards.
When the series was produced, only two volumes of the manga had been released along with a few chapters in monthly magazines. Much of the plot of the anime was original material.