The scratches on the actual housing may be difficult to take off with displex. Displex is only for the LCD screen, but that being said, many people have had some horror stories using displex. If you use it you have to be very careful and use it right.
As for the housing, I do believe it is easy to take off the old one and put a new one on.
There are threads here and there as to how someone did theres, but here is a link that was floating around the forums: RIM Blackberry Forums - Pinstack.com - Free Blackberry Downloads - Device Photos
You have to register to download it, but it gives you step by step pictures of how it is assembled, so one can say you can figure out how to take it apart