Dragon Knight - Classic Build

would it be good if i use mjollnir?

Always get Dragon Tail at level 1. It's meaningless to get Breathe Fire, it's simple math: you'll deal 90 Magic Damage (67.5 to Heroes, even worse) and on top of that, spending 100 mana, which is half of what you start with. Definitely not worth it. Getting the stun will 1) save you from first blood attempts in a 2v3 or 1v3 early gank 2) get a guarantee first blood in a 3v2 and sometimes in a 2v2 if your mate have a nice stun like Vengeful Spirit.

Not to say it's the biggest level 1 stun in the game (2.5 seconds, this is a steal, making DK that much powerful and feared right after he step out of the Fountain).

Item build is fine, except for Vanguard. He doesn't need it. With that much gold you could get better items, like half way through BKB (Ogre Axe alone will give you 190 HP for 1k gold).

Just my 2 cents though... not saying your build doesn't work!

Cheers!