With this size model railroad, the cost of buying the large amount of track required is a concern. Luckily there are sources available today that were non-existent when I was a kid, namely the Internet. I will buy as much track second hand, primarily eBay, and leave buying new track for the key switches that need to be reliable, i.e. in the classification yard and mainline switches.
I am considering buying and using code 80 track for the hidden area; this will be less derailment prone for this area, as the rail is higher. This will, of course, require transition areas on the connections to the remaining code 55 areas.