Field Marshal and Ferd45231:
I am very interested in your project. I have little background in that era, but I do know some good paper model sources, with the advantage of them being FREE.
Lets start here:
link
German specific, OK the instruction are German as well.
Scaled for model railroads, so HO (1/87) and N (1/160).
So you will need to rescale, I suggest "GIMP" which is a FREE version of PhotoShop. I can go into details if needed.
Next is Projekt Bastelbogen:
There are a number of collections and scales, but almost all are vintage German style buildings:
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Another great start is PaperMau:
link
A site for free paper models, I selected buildings, but will be in all scales, locals, styles.
If you want buy items:
It is difficult to get 1/56th scale for 28mm figures:
This is one set, but really older style buildings:
link
There is the Reichstag:
link
But in 1/400th scale
A Brandenburg Gate, but N gauge (1/160)
1999.co.jp/eng/10629393
You will have more luck with industrial areas than the Neo-Classical government areas or even commercial areas.
Which I know there are more out there from this:
theminiaturespage.com
"TMP link