To begin with Hellsing is a manga series which is written as well as illustrated by Kota Hirano, and after it gained enormous success it was adapted into an anime while the confusion between Hellsing and Hellsing Ultimate began to rose. Hellsing anime have been produced by Gonzo and it is directed by Umanosuke, it has been highlighted that the first half of the anime is adaptated from the manga and rest of the anime is all exclusive original creation. Talking about Hellsing Ultimate is an OVA series which is produced by Geneon and as well it is produced by the Satelight while staying true to the original anime so, there is no mix up.
The first Hellsing series came out in 2001 which included the first half of the manga and was about 20 minutes long each episode, it gained immense popularity when it first came out. The second half of the manga is created aa the manga was still incomplete so they have to create the second half without the help of manga.
Hellsing Ultimate anime series started from the year 2006. It constitutes of 10 chapters which is about 40 minutes long. It explains the story in a very detailed form without any letdown. So, both the Hellsing and Hellsing Ultimate are different due to the storyline being changed from tge original manga. Coming to the differences between both Hellsing and Hellsing Ultimate as Helling is produced in 2001 it doesn't have that good animation style that Hellsing Ultimate have but the soundtrack is definitely unique of Hellsing.
What makes Alucard so powerful in Hellstring?
Simple answer to this question is that Alucard's real identity is of Count Dracula which makes him so strong. As well as in Hellstring we are well aware about the fact that Dracula is very powerful. In Alucard's life once he was Vlad the Impaler, the Ottoman's boogeyman. So, in the name of the God he slaughtered a lot of people to uphold justice.
So, after that Vlad got captured eventually and was sentenced to death which is very similar to his real life counterpart. When he was about to die Vlad licked the blood that was present on the floor and then he became the vulgar creature which we know or call as vampire. He became despicable in nature and rejected god and at the end he became a vampire.
He became the hunter of God's children and the creature of filth. Unlike many other vampire's he was not bitten but he was made on his own by himself. For the information Blood is not only regarded as a fluid but it is the basis of every life without blood running through our veins we can't imagine our life. It can also be said as the soul of a person.
When one vampire drink or sucks your blood so he is not drinking the blood but also sucking your lifeforce from you making you older and keeping himself young, fit and fine by expanding his own life force and exhausting your life force
Alucard is different from the other vampires as he had drink the blood of over than hundred or thousand's thus giving him the title of Fortress. He had consumed so much blood and soul that it had made him very powerful and also almost an immortal. So, his power level is definitely very high.
Alucard have some restrictions on him due to his extreme power level. For example, they put a handicap on his power by putting a handicap on the blood. During his basic level when he is not emitting a lot of power during that time he is almost unkillable as he gains super regenerative powers which let him regenerate him almost infinite times. This is also due to the fact that he is not consuming a lot of blood just a minimal amount of blood.
When Alucard reaches level 0 during that time he became very powerful but at the same time it also makes him vulnerable that he may get killed too in this state. We can say that it is the side effect of using all of his blood in one go leaving no blood to even protect him. So, power up comes with a flaw.
Alucard can be said and he is really the real vampire as he became a vampire out of his own without anyone's help. He choose the path of filth thus he became a true vampire rather than sucking the blood of dome random being. So, there is a difference between Alucard and other vampires.
Also, he consumed a lot of blood literally from the armies that he fought against and then becomes almost immortal. But, on the other hand there are some people who don't regard him as a vampire and that is due to the fact that Alucard is a whole different kind of monster. We can say that Alucard is a sort of vessel which contains a lot of soul and blood which he can use whenever he wants. The world that Alucard dwells is a world based on biblical facts rather than something supernatural phenomenon.
So, some people thinks that we should not call him a vampire as he drinks blood of other beings. But just because he drinks the blood of other beings we can't label him as a vampire. He is much more than a normal vampire. It is also the case when no restrictions are lifted from him he resembles very much of vampire but when he lifts those restrictions he becomes something else.
How, Seras turned into a vampire is completely unknown as Seras is not converted due to Alucard. Also, Alucard never ever created ghouls after drinking someone's blood as a normal campire would do. So, he is definitely an other monster.