Most important thing is to make sure it isn't damp and that the air can circulate, so fix any holes in the roof and consider adding an air vent.
Mesh windows are a good idea for the daytime, you can make wooden or perspex shutters for night time.
If the rabbits are to be allowed to free range inside the shed you will need to clear away any hazards such as garden chemicals and tins of paint.
It is a good idea to swap the door for double stable doors to prevent escapes.
If you have electricity you might want to add a frost protection heater, either a panel heater or one of the tubular ones. Obviously you need to make absolutely certain the rabbits cannot chew the cable xx