Without knowing what the geometry of this part is like its difficult to recommend exactly how to approach modelling it in ProE but I've dealt with similar problems so here are a few general points:
I'd suggest either 1/ using a copygeom to copy in the original surfaces or 2/ create an assembly and have the original and your new model sitting on top of each other with the parts in different colours. You're going to be constantly comparing between the 2 so you want to make this as easy as possible.
You need to pick out some key feature on the original model that you can then use as your main references when constructing your ProE model. If you want to make a decent job of the engineering aspects as well as the modelling then you'll try to relate these references to how the part is going to be manufactured and select references that can be used as the main datums throughout manufacture and inspection.
I'm guessing that your plastic part might have highly contoured geometry that doesn't lend itself to simplistic extrudes, revolves etc. ProE's Boundary Blend tool is very powerful at creating contoured surfaces and can control tangency etc to adjacent surfaces. It needs a framework of curves that define the surface perimter, you can also add multiple curves within to control how the surface flows. These curves can be 2D or 3D as required, you can also work with splines which can be very useful as these don't generate individual surface facets from every element of sketched curves.
I'd suggest placing a few datum planes through some of the key/dominant parts of the geometry then constructing curves on these planes as the base features for your Boundary Blends. If the original geometry is too complicated to follow using sketches you could place datum points then use splines to join them up but this will be much harder to modify in the future.
To generate the 2nd direction curves (that should make sense once you've used boundary blends) you can generate points on the first direction curves then reference these with the 2nd direction curves (Boundary Blends can't have gaps between the curves)
Its difficult to explain Boundary Blends in any more detail with a short number of words but if you've not used it before I'd recommend learning about it, try the PTC University class on it, if you're using a genuine copy of ProE you should have access.
Finally, if your "Industrial Designer" is some type of styling guru who will throw a hissy fit if the engineering team dares to distort the shape of his wonderful creation by so much as a micron then you'll need to educate him a little in the requirements of manufacturing, inspection etc. Its easy to end up with arguments between Stylists and Engineers but if you explain to him the constraints of manufacturing and the need for things like draft angle, tool split lines etc then its usually possible for the two functions to work together,
