On the Xbox, it was impossible to get the house in Windhelm without starting the main quest, because you had to talk to Balgruuf, but Balgruuf wanted you to kill a dragon before reacting to the Stormcloak quest-line. I read in one of the patch notes that this has been addressed so if you back out of the initial conversation with Balgruuf and then do the Stormcloak quest, all will be fine, but I haven't tested that since the patch.
Anyway, you don't have to do much of the main quest to get Breezehome, just slay the dragon.
That's a pretty big quest for the main line. Completing it means the world will now be filled with dragons, which to someone trying to avoid the main quest, is a big deal.