I was contemplating doing this one at a later date, so I'm glad you will do it sooner (and better). To really do it right, I was going to do the Kroll Opera house, "Himmler's House", the Moltke Bridge, etc- all of the major land marks. Other buildings could probably suffice, but why not go the entire distance if you are putting one major landmark?
I would do it as maybe 5 separate pieces.
This book is an indispensable aid to understanding how the battle for the city played out:
http://www.amazon.com/Bloody-Streets-So ... 1906033129
There are thought to be 2-300 defenders (rather than the 1,000 often quoted) of the building (primarily in the basement), with localized counterattacks being made from there. The book above gives as detailed a picture of the fighting as I have yet read (and I think I've read them all by now).