First step, I'd contact the municipality to verify that it's even
permissible to do what you are thinking about. Many areas won't allow
people to fill in flood plains, or it may be considered wet lands and
have environmental restrictionson what you can do. In many of those
cases, the engineering studies, environmental studies, permits, etc,
just to find out if you can do what you want to do, can take more time
and money than the actual fill work.