it's a free DLC,i guess 2nd DLC have another missions with vehicles like maps or arenas
No telling if it's a free DLC, actually. 2 Launch DLCs are free, which are Normandy Crash Site and Zaeed. As for Hammerhead, only time and greed will tell.
This is the picture of the M57 Hammerhead (the only one on the entire web):
img198.imageshack.us/img198/3341/newmako.jpg
It isn't from any official source, so I can't promise it's just the way it will appear in the game, but I doubt anyone would fake it either. Especially days before the release of the game.