In workflow generate a mesh. Do it on high for the maximum number of faces.
After that is done run decimate mesh from the tools menu. They key is choosing a suitable number of faces that is the balance between detail in the model and file size. I guess that depends on what you are modelling and how big it is.
I would say most Surveying software packages run on regular computers don't really like more than a million faces. and if you can get it down to 250,000 it would be better if you are not loosing too much detail for your purposes. You will have to try it and see.
Then export it as a .dxf.
Don't try and generate the mesh with a smaller number of faces at the first step. The result is not the same as generating the mesh on high then decimating to the desired number later.