I've never yet seen fixed price work well, and I think on something of this size it's doomed to fail. The biggest problem is defining the scope which is almost

I've never yet seen fixed price work well, and I think on something of this size it's doomed to fail. The biggest problem is defining the scope which is almost impossible to do in any detail at the beginning of a project. Instead, I'd recommend either fixed price for small, bite sized tasks, or (as per my current contract) a weekly rate until completion. I work on rapid turn-around, so the client almost always knows exactly where the project's at and how it fits with their requirements. Of course a precondition of this is a decent level of trust on both sides, and a client who's not a pain.