Currently at the part where I assault the vampire stronghold with the Dawnguard. I just want to know if these people are essential or if I should be looking out for them.

they are essential

Everyone in Skyrim is essential; haven't you heard?

I haven't heard about the word. Because the Dawnguard is the word.

No, they are not. I had 2 members of Dawnguard die in the assault, one of them in Lord Harkon's bedroom upstairs, and another just inside, killed by the blacksmith.
Both of them were just really basic, generic guys, Celann and Beleval. Both of them were potential followers.

Everyone is [censored] essential.