I get handed Solidworks CAD files from industrial designers that are a complete disaster. So much so, that I often use their CAD file only as a reference to what the end result should be, and make a new CAD model to match. I am doing top down assemblies, with multi-body parts. These are injection-molded plastic cases. The I.D. creates a single body model of the full assembly. The problem is this model has a feature tree that is the CAD equivalent of "spaghetti code". There are multiply times that the entire model was scaled to change size, other versions of the design were brought in and sections of the two were combined together. It was all modelled in the quickest way possible, which makes it almost as useful as if it were modelled in clay. 

In the past I have been able to use the sketches of particular features to find their dimensions, and recreate them. That is difficult to impossible with this model. I need the model I create to be able to be altered as the design progresses, so copying surfaces and such will likely just cause problems down the road. I also need to create the basic shape so I can split it and shell it. There are many features that would not shell on the I.D. model.

TL;DR

What are good methods for creating a mechanical design CAD model from an industrial designers model. 

Thanks for any help!