If your going to replace the whole of the floor get the waterproof chipboard from one of your local building suppliers. It has a sort of green colour to it and has tongue and grooved edges to it all round.
I hope we can patch it as it goes under at least one internal wall
I shall talk to the chippy and see what says, good chance to see if while he's doing one job he'll do another 'small one' and do my baseboard bracing 8)