Yes. Rear mounted engine
Now I have three sources with three different designations of the bus:
1) Mercedes L 608 Heck Clubstar
2) Mercedes Clubstar Heck
3) Both of them in the text
At that time, to my knowledge, two 80 HP engine types (gasoline and diesel) were available by Mercedes for the L 608. I couldn't find out, which variant was used on this bus.
In my opinion, this puzzle is solved. You are invited, to make a suggestion for the correct designation.
FOR AN ADDITIONAL POINT: WHO DID THE CHASSIS CONVERSION (engine from front to the rear)?