First, do a google search on the 88 to see its typical emplacement and if it would be in an urban setting. Then see if yu can find some period pictures of WW II urban settings to get a feel for what damage looks llike. I would use other models as refernce in the same way I'd use a novel as a reference, i.e., with a huge grain of salt.
Second, chalk the purchase of the sandbags up to experience. They are really to stiff to use and never look realistic. Pick up some epoxy putty at your local hardware store and make your own. They'll drape much better and look like real sandbags. Commercial sandbag sets are also way too uniform in shape and size.
Third, determine the "when" of the diorama. This will determine the colors you'll be painting. Before 1943, panzer gray rules. From March 43 on, dark yellow with red brown and or dark green disruptive patterns were the norm in Europe.