Dracula is the gothic novel upon which countless adaptations, including to some degree Hellsing, are based.
At the time the novel takes place, Alucard, or Count Dracula as he was called then, was said to have a "child-brain" by Professor van Hellsing, a reference not to lack of reasoning ability but only his ignorance (at the time) of how far his powers extended. This is possibly was what allowed Helsing to defeat and dominate him in the end. However, even at the development stage, it is implied that the Count Dracula was incredibly powerful, as shown by Walter's comment during the invasion of Hellsing by the Valentine Brothers: "Compared with what the first Sir Hellsing had to deal with a century ago, this is hardly what I'd call a pinch." In the novel his powers included the following:
-The ability to drink blood and grow stronger, like a leech, as stated by John Harker, as well as the immorality that comes with it. (although Alucard is not immortal he is very hard to kill as Integra Fairbrook Wingates van Hellsing states in the ova "take off his head, cut out his heart, blow him too bits! do you really think you could kill a thousand year old king of vampires.")
-The count as well as his brides can turn into an immaterial dust then reform (often into a swirling image with red eyes). Dracula is also able to become a black mass of shadows with red eyes intersperced throughout, which then is changes into a large pack of rats.
-Is able to go out during the day but in a weakened (semi-human) state.
-Can make fog, as well as other natural elements, to some degree.
-Can change into a wolf or bat, as well as command them. These are his restriction levels (he can aslo turn into his different bodies he has used over his life time. As seen in the ova fight with Walter)
-The ability to hypnotize people, usually his prey, to some degree.
-During a briefing Van Hellsing tells those involved that though a vampire's physical strength is not infinite, it is great enough that one should avoid making any contact with them.
In addition, at one point his costume is said to include white gloves, and his personality, as revealed by the patient Renfield, paints him as an (often smiling) man of action rather than words.
At one point, after Lucy Westenra's "death," Helsing begins to laugh, and when questioned explains that he laughs at the fallacies of others "Because I know." The monologue, in that the character explains their apparent insensitivity, is similar to the speech Alucard gives to Integra, "Because I am a monster."
The image of Alucard kneeling over the ground and licking up the blood is probably inspired by an identical stance taken by his servant Renfield at one point in the novel.
As said in the manga during Alucard's return to London, he previously sailed in on a Russian Ship known as the Demeter. In addition, in a flashback van Helsing, having defeated Alucard, made mention of the mark of a communion wafer on "her" head. This is a reference to Mina Harker, whose blood was taken by Dracula and who took his blood into herself, the sign of which is shown when a communion waffer (a holy object) touched to her head burned it. Mina's remains were later acquired by Millenium and used to create their artificial vampires.