Hi Robert, try Scarm software, its simple to use and its free. http://www.scarm.info I'm not sure if its available for mac. Also try http://www.freetrackplans.com you will get some good ideas from there.
I have built a number of layouts in sheds, if you want to make your layout removable build it on a sheet of ply of MDf in sections, then just drop them into place on your layout. Check out my page in the services section to see what I mean.
Best bit of advise I can give is research, research, research!