For a general overview I would recommend looking at dune-buggy frame plans.
Building your dune buggy. Custom building instructions for you!
You could take an existing design and alter the layout for the wheel size, ride height, and interior configuration you're aiming for, or just use it as a reference for how the structure needs to be laid out to handle the drive train and suspension element stresses while protecting the occupants.
If you want free Solid CAD try Alibre Xpress
http://www.alibre.com/xpress