Is this switching off centering and resizing just a Pro thing? Can't find it in 4.

I've used AutoCAD imports into Vue 4 extensively in my work. Try the following

1 - duplicate the imported model
2 - enlarge the duplicate slightly
3 - use the enlargeed duplicate to create a boolean difference in the terrain
4 - place the imported AutoCAD model within the slightly enlarged depression
This doesn't work every time and is a real pain with large models as it slows down the PC. If possible, create another object in AutoCAD containing just the parts of the model that will be in contact with the terrain. Use that for the boolean operation. This can reduce the polygon count significantly.

My preferred method is to surround the building with lots of carefully placed smaller terrains and make extensive use of the brush tool in the terrain editor. This "hand crafted" approach gives the best look to the finished image and, with practice, is just as quick as messing about with booleans.

Hope this helps.

Cheers,
Mark