How do I start Dawnguard if I want to be a vampire?
Also do vampire perks cost levels?

vampire perks do not cost levels, as you fight in vampire lord form you will not be leveling you base form but rather your vampire form.

To become a vampire lord in the dawnguard, don't become a vampire at the start. Follow the questline, join the dawnguard, and it'll progress to a fork in the road where you choose to join the vampires or reject them and go back to the dawnguard.

^This, plus if you are already a Vampire before you get Dawnguard and want to join the Volkihar there's no penalty.

TY all