1. Did most buildings have basements?

2. What were they like? Rubble walls, cinderblock, brick?

3. What about the floor? Slab concrete? Dirt?

You have to remember Berlin was a very old and cosmopolitan city. Most buildings did have cellars. Typical construction was plaster over a brick base, usually 3 to 4 stories high. Of course, there were various government building with Roman columns (that's what I'm planning for the Berlin diorama I am working on...), and store fronts too.
http://jtrowbridge5.smugmug.com/Battlefor/Battle-for-Berlin-1/19890640_gdx7cc#!p=1&n=10
Check out this website, it's got some great pics, and you could always do a quick google search that will provide lots of pics.
Hope this helps.

Cheers-
Bob M

You can also check on Google taping "east Berlin".
There are still many buildings which were left in the war state during a long period in the east area of Berlin.
I remeber clearly seeing a lot of old facades with still shells and amo impacts left on them in the 90s'.
Take in account that the communist government didn't cared a lot about old buildings and put more moneys in the party symbols within the brand new buildings, some were actually destroyed at the dawn of the new millenium.
For instance :



So have a tour on Google and you may find some ideas