I'm in the process of building a sectional layout using a benchwork adapted from a system used by a friend of mine.
When you say "modular" do you mean that the various elements would be interchangeable? Or simply that it could be easily separated, like a sectional layout?
You can make a layout that is easy to take apart without spending a lot on pre-cut modular benchwork.
One approach would be a sectional layout like the Model Railroader Beer Line layout that ran from January 2009 to June 2009.
Another would be one of the simpler shelf layout designs in the new Iain Rice book on Shelf Railroads. Some of those designs use benchwork that essentially hangs on shelf brackets you buy at the hardware store.
My layout breaks into four sections (I'm just building the benchwork now; no track yet). The framework of each section is made of 3/4 inch ripped plywood for the cross pieces, 1/4-inch (you read that right!) ripped plywood for the side rails, and 1/4-inch plywood sheeting for the top surface. 2 sections are 3-by-4 feet, and 2 sections are 6.75-by-2 feet. I wanted to break it into sections so it would be easy to take down and move, which we hope to do some day. I also wanted sections so it would be easy to remove the layout from its main room in the basement in case we put in new floor, ceiling and lighting there, which we can't do right now ($$$) but would like to some day. I didn't want to put off the railroad until I had the big bucks for that future project.
You can see more about my layout at my blog, bloggingontherr.blogspot.com.
