I join G3750 and mwb in their advice to you regarding reach-a-bility, if you will, and I agree further with mwb's caution about being able to stand on the layout.
When I built my layout, which accrued over a 5-year period, with ever-expanding benchwork, though I began against the walls, my eagerness had me going back-n-forth with changes for many years, right up to the present, walking atop the layout everywhere. I was glad I built all levels of the layout of 2x4's and 3/4" plywood.
Also, several vignettes throughout the layout are actually portable dioramas so that I can remove them from where I need to step without clipping something with my foot as I work on re-crafting an area.
For example, considering this smaller wing of the layout, most of it is within reach of my standing in the aisle, except for that second-level loop that requires removing some structures on Level 1 to accommodate my mounting the layout and reaching it from the aisle....
...most of the layout is like this area, which is not within reach from any aisle...
...so I am glad I built the entire shebang like this section, out of 2x4s and 3/4" plywood to allow my walking atop it....
P.S. You may also want to consider what happens if you have derailments. How will you reach them? You know they will occur (usually at the furthest places from you, especially in tunnels - a favorite place for them to happen.
Take your time an enjoy the creative adventure, George. There are none quite like it, IMHO.
FrankM.
