Reading what I'd written, it didn't make sense, apologies.
-For anything structural (side panels, external faces, wheel base panel), 18mm would be recommended.
-For the cockpit base panel (footwell / pedal mount), you could get away with 12mm MDF but I would use 18mm anyway mainly to allow the use of threaded inserts for bolting any pedal mount down.
-For aesthetic parts, any thickness will suffice. I like to use double thickness (18mm external, 12mm internal) to allow inserts/windows to be added.
The pedal mount is sturdy using the CSPs. There is a little bit of play in the seat runner it's mounted on though (not really noticeable when in use).