Slow and steady wins the race. If this is your first large layout then some good advice has been posted already to start small. Get some bench work up and track down and run some trains. Just build what YOU want - almost like a modular layout and keep adding sections when you are ready.
If you don't want to do backdrops on the sloped ceiling then at least paint it sky blue with some clouds. Adds a lot to the scene in front of it.
Bob
