Document case? If it's the black case on the side of the fuselage above the oxygen cylinder – that's where the oxygen mask was stowed. There is a copper pipe coming up from the oxygen cylinder to the regulator – the small panel with dials on the side of the fuselage. Presumably there would have been some kind of hose connected to regulator, the other end disappearing into the case.
The construction of the oil cooler is standard on Part's PE sets. Best to make a jig out of a couple of vertical L section pieces of Plasistruct glued to a base. Helps to keep all the pieces aligned.
Don't like the windscreen though. I would prefer a PE brass frame with clear film glazing.
Yahu have just released a prepainted instrument panel for the kit. I think you really need to get one.
http://www.jadarhobby.pl/yahu-models-yma3202-132-pzl-p11c-silver-wings-p-46180.html