I want to build an East German town circa the late 1950's or early 1960's, preferably with a Communist Party office in it. I assume it would look much like a 1940's German town, but what does a Communist Party office look like?

Well, it would look as 50-s or 60-s German town. Nothing extra specific.

http://www.ddr-fotos.de/gdr_photos.htm

As Roman pointed out... It would look like any town from that period. If it was in a bigger city it would probably look like a big bulky concrete monstrosity but in smaller towns and villages it would be located in any old building and the only thing you could see that it was the local partyoffice would be the SED logo or the DDR logo on a plaque next to the door.