Look at the Model Railroader database and at the Micro-Trains plans for
their Z track. Z-scale is actually about 2/3 to 3/4 (.7272) of N-scale
and N-scale plans adapt well to Z with only minimal adjustments. This
will give you room to add larger scenic areas and features like small
yards. Have seen a few very good L-shaped Z layouts recently but corners
make difficult areas for placing scenery. Consider Z Track modules if
building a larger layout.
Using foam - I have built both N and Z-scale layouts with the dense pink
or blue foam, and they work well. After shaping it I cover the surface
with drywall compound in 2 or 3 layers. Other material like paper mache'
and premixed tile grout are also good. It is also easy to fill gaps in
the foam with "RightStuff" expanding insulation foam and then trim or
shape the excess after it cures fully. Use it with gloves as it is
sticky and messy and does not come out of clothing or carpets
(experience speaking here).
For grades I also use Woodland Scenics precut foam grades and build
foam scenery in and around it. For gluing foam Woodland Scenics glue is
about the easiest to use if a little pricey. Pin or weight the foam in
place and it will be fixed in about 4 hours. Non-solvent based cartridge
glue for projects is also suitable. Have been disappointed in Alene's
Tacky glue and the blue type designed for foam as it is too stiff.