How many chances do you get to become a Vampire Lord

So, I am working on a new playthrough of Skyrim with the two new DLCs and I would like to work in being a vampire lord early into the game. Doing a bit of reading however it seems I have to select to be a dawnguard if I want to be able to craft crossbows and bolts... which I do want..

Can I choose to join the dawnguard up front and then become a vampire lord later down the road or is this the only time this option will come up?