Toby, best advise I can give and you probably know this is know when to let the pro's handle an aspect...namely the footings and foundation. This is the most critical part of your building and if not done right will only lead to many problems as the structure goes up.
As far the building on top of the foundation, you can have at it and mistakes are easily redone or covered up, but done let the foundation fall into this catagory-you need an excellent square base to start with proper drainage.
Other than that have fun with it