First thing is solve the water intrusion problem. You may have to build up a berm outside to redirect the water, then for the floor, have a few truck loads of crushed concrete or rock brought in, so you can build up a base. Then you can decide if you want to pour a slab or not later. As for insulating. You could float a wall around the inside perimeter, but I would do it after you get a slab down, so that it will be easier to do it.
Then again, there is always the possibility of taking 4x8 sheets of play, put 1 or 2 inch XPS on it, glued, then hang on the members. The seams you can just use 1x4's to place over. Those are the only couple of things I can think of.