Breton makes a very strong choice as well due to their significant Conjuration boost as well! Given it's the Bladesman's main skill, the Familiar summon means you can start leveling right away!

I'm already level 33 with this build owning all the baddies with Serana, totally keeping a leering eye on her.

so i was thinking of starting this but i already have a necro char so could i go with the different summons in dawnguard and such until u can get a storm atronach?

The Dawnguard summons are a much better option that atronachs in my opinion, able to fill all 3 roles (magic, ranged, melee). Once you get relevant perks they're alot tougher than atronachs (though the Wrathman is still beaten by a Dremora Lord in terms of damage) and pretty much on-par with them for damage.

ok yeah i am not planning on going to perma summons but all the necro perks affect the dawnguard summons so grabbing boneman at level ten(or earlier if i trek to the castle) and also arinels shade is a nice choice as its almost immune to physical damage. summoning arvak to ride around on also seems fun and makes sense with the character.

Join Dawnguard or Vampires?

(Just want Serana. Obviously, Dawnguard has no bad effects, but maybe being Vampire can complement?)

Doesn't really matter. Neither of the factions really has any exclusive equipment that complements the build in any way, and the Vampire Lord abilities don't really add anything. Vampire bonuses don't do much either, though it's easier to get the Soul Cairn summons on the Vampire side (you can buy them instead of having to search around).

I'll go Dawnguard side, I can get the Soul Cairn summons anyway, and there's no downside effects. Like a said, Serana is a pretty good follower in my opinion, and on Dawnguard, I can still get her, right?

Yeah the quests on each side are surprisingly similar, you get Serana on both (if you talk to her about curing herself at the end of the questline you can get her to cure herself)

Ok, thanks for the quick replies.