I'm a huge Dragonball fan, in any of it's forms.
I'll answer in order:
1) As with any DB game, the campaign is just fighting. If you loved the plot of Dragonball Z/GT you'll enjoy this. The fights themselves are all very samey, but enjoyable.
2) The online multiplayer is, from my experiences, laggy as hell, and these days not very active at all. it is just standard fights (ranked or unranked) or a World Tournament mode. If you organise games with friends it is fine.
3) Short answer: no. Basically just picking a costume and hairstyle, as well as changing some colours. No matter what, your character will end up looking like someone that already exists, in a different pallet. Make of that what you will, but it was not enough for me.