Cleaning up and preparing a building model supplied by a client. Looking for advice on workflow.

I have a games and photography background and Im looking to get into arch viz.

To this end I have been given an Archicad file from a potential client to do some experimentation and learning on with a view to doing some work for them in the future.

What is the best way to get a model from Archicad (.pln) to UE4?

I have found exporting to .fbx and then importing to Maya allows me to retain the materials from Archicad.

However the big problem that I have is the state of the mesh. The topology is so messy that it is unusable as it is. There is geometry stacked on top of other geometry (causing z-fighting) unconnected faces and loads of hidden geometry in the walls (insulation I think!).

Ive started cleaning it up but honestly I think it is going to be quicker if I just rebuild it myself. If I do this Im thinking that Ill use a combination of the quad draw tools (in Maya or possibly 3D coat) and manually rebuilding and then snapping the verts to the supplied model

Can anyone suggest a better workflow?

Is it common for Architects to supply models with bad geometry? -if not is there anything I do to encourage my (potential) client to create cleaner models?

Any advice or tips gratefully received!