In answer to your questions, there is no difficulty setting for the story mode and you have to play story to get characters; that is the only way to unlock them as far as I know .
As for whether or not to get it, I can only offer my own reflections of the game. I found that aside from the new characters it wasn't much different from Storm 2. The survival mode got extremely tedious and the tournament mode was sloppy and poorly put together. I felt the game makers just slapped half a game together and put as little effort as possible into it. Also, player matches now record wins/losses, which is annoying because you can't get practice in online anymore without ruining your win to loss ratio.
Also, the new substitution system is terrible. Basically you have a bar on the screen with four substitutions, using the LT to automatically get out of a bad situation. At first I thought this was great since it would get rid of the spammers online, but it turned out to be extremely aggravating for me since I found that everyone just runs away at any sign of danger. If this was only online players it would be bearable since I expect people online to play as cheaply as possible, but the computer plays the exact same way too. Constantly running away from everything. Or you have people online who just rush you constantly, force you to use up all your substitution points, then punish you with multiple combos. It got annoying really fast.
I loved Storm 2, but this game is not worth it. It's the same game with a couple bells and whistles added and is marred by some really poor gameplay choices. I would not recommend it, especially not for $60. Oh yeah, if you're also concerned about achievements, it's an irritating grind fest, especially for the info cards.