The Tschakos look great with the proper matching Police P.08 pistols in your display case. This headgear is a great idea for a Police Luger display.
Since your Gendarm was a member of the SS, as indicated by the runes below the pocket, you could certainly use the SS brocade belt with Police (or SS) buckle. He would have been authorized to wear either belt. A 1st Class KVK with Swords would be good on your uniform. The 18 year Pol Service Medal should have a white eagle embroidered on the ribbon though so you are stuck with the 8 year medal on this bar since it has the proper ribbon for that award.
If you have a Volkspolizei P.08 you could also look for a VOPO Tschako for a display. Here is an enlisted version of the Tschako. The uniform is a Model 1948 Volkspolizei NCO uniform and has a VOPO hard shell luger holster on the belt. I always like to see accouterments with displays and you have a flair for making them.