I feel like I am reading conflicting information on this...

I would like to join the Dawnguard, but still be a Vampire Lord.

I am currently at the part where Lord Harkon offers to turn you. I know that having him turn me sides me with the vampires. This I do not want.

It is my understanding however that Serana can turn me later on during the Dawnguard questline. This I like... But have also read that as a vampire, I cannot continue the Dawnguard questline. Is this latter part true? If I let Serana turn me into a Vampire Lord, will I be unable to continue the Dawnguard questline?

Correct. If you become a vampire in any form the dawnguard will not talk to you unless you cure yourself plus becoming a vampire lord on the dawnguard side is a nerfed version since you won't have the buffs and items that makes the vampire lord truly awesome. *looks at rings and blood chalice*