can a level 5 beat the DLC ??

You have to be Level 10 before the Dawnguard main quest is offered.

Attacks start at level 9 and the questline starts at 10. It's not true, the attacks only occure while you are in the area, you can complete the questline and just avoid towns at night. From my experience the attacks mostly (if they don't bug out) happen between 8pm to 6 am.