Go big-it will give you more modeling options, or space if you get out the hobby for some reason. Don't want to kick yourself later, regarding shouldhave/wouldhave/couldhave...
Regarding the trackplanning(or CAD) software, I use the old Abracadata's 3D RR CAD. Do you have a CAD/drafting background? What I use has a smaller learning curve than some others, + plenty of libraries of both track, scenery, and structures. Any sort of CAD program can have quite a learning curve, if you're not familiar with CAD, or drafting in general...