I'm assuming the contents of the catalog in the free version is pretty minimal - but have you got a pdf of what's in the full catalog?
The catalogue as shipped is quite bloated(50-60MB) - most users cull it quite radically to suit their work methods (& speed the system up)
If I want an unusual wall size (250mm Hebel blocks), will I be able to change/create a 250mm thick wall to draw with? Will I be able to make 800x810 windows?
Yes, & yes - both are very simple to do
If not, which 3D formats does it import and export? What formats will I be able to export my finished drawings in? Something AutoCad can load?
Import 3D dwg, dxf, 3ds & skp
Export 3D dwg, dxf, 3ds, wrl, opt & lp
Export 2D dwg & dxf
Do the roofs work any better?
Than what? The roof tool is very powerful - takes some practice to master it