@Joseph F. this would be a very big project and I would not suggest it with out maybe partnering with some one that was very experienced. House fires can damage a lot and you would have to have a engineer look at this project to see just how far the structural damage has gone.
As far as is it worth repairing. That is where you bring your comps, the engineers requests, estimates on repair, and your holding cost numbers together and see if the deal works. Real estate is all about numbers. If the numbers don't work than no its not worth it. Without seeing the house my self and having the other numbers it would be hard to say.
As far as building new that is a good idea but it all depends on comps and the neighborhood. If the comps support a $1.1m-1.8m. home then that would be the range to be in. There are a lot of factors that play into this though. Is this an older neighborhood that is historic? If so the city may not allow for you to just tear it down and build new. What would be the cost to tear it down and clear the lot to build new?
Coming in with no experience and attempting a job this size would be hard. There are many factors that could go wrong and leave you stuck with a costly pile of nothing.
Yes you would need to get estimates first for both directions so you could run your numbers and see if the deal works or not. When you have these numbers then you can sit down and decide if that $550k is too much or if its a good deal. Without those numbers it like trying to shoot a target with a blindfold on.
Hope this helps, Clayton