Hi Mike - My advice is when you are building a large ambitious plan such as you are proposing is to break it down into several smaller self contained projects than can be completed in a reasonable time, most people (myself included) underestimate the amount of time it takes to build a large layout, not to mention the costs, timber to build the boards, track, electrics and scenic materials all add up. I had a house with a large loft about thirty years ago and set about planning to fill it. i did not have a plan how each stage was going to be completed and eventually ran out of cash and enthusiasm, result the layout never was finished and was eventually abandoned in favour of a smaller layout in the spare room.
That said, best of luck, I do like layouts with lots of operating potential, you can't beat playing trains after all!
Jim