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Does anyone have experience using World Machine or World Creator with Sketchup / Enscape?
Or will I have to go to a more robust modeller?
SketchUp can deal with it - the biggest issue you will have is that you'll need to hide the edges in the geometry for detailed terrain, it will become pretty painful to work with pretty quickly.
The biggest issue is that you'll want to have the meshes tiled - having to deal with one super massive textures and one giant lump of geometry will be tough.
I tried importing a 1024 resolution chunk from WM and it was ok in both SketchUp and Enscape