Cool, so I'll work at buying a house in Solitude. Also, do Bretons generally support the Empire?

Generally, but the rural ones are fiercely independent and clannish.

So, my guy will remain neutral from the war but, in the end, join the Empire as a means to end prolonged suffering. Plus, since the Stormcloaks would only accept Nords really, he couldn't join them. So, why might he become an assassin, do you have any ideas?

i had a character in oblivion that his class was Dark knight he was more a lord of destruction that knew how to talk with people

As a Stormcloak supporter I am obliged to say that this is false

Maybe he became an assassin because he was finally wanted to do something about all the corruption in the world, having seen a relative hanged for a crime they didn't commit.