Hi, i'm currently level 15 but not sure when i should embark on the dawnguard quest line? is crossbow scaled etc?
when did everybody else start it and when is the ideal time?

Devs say level 10, I tried, but Gargoyls and some types of vampires are tough to beat.

My advice? Leave it till around lvl 20 some of the enemies kick serious butt !

cool, i was just worried about losing on some gear that will be obsolete when i get there.

The crossbow can be improved well at grindstones, especially if you perform all the Ancient Technology quests. I started Dawnguard around level 17. Now I'm level 51 and the armor and weapons you find are still totally viable (when improved via smithing).