Gyprock/plasterboard/drywall would be the best way to cover the ceiling portion of the room Easy to work with for one person, lots of dust though. It also provides and extra layer of insulation to the existing batts.
The lowest part of the sloping ceiling, where it meets the floor, is mostly unuasable as its low height make access difficult. Do as mechatrommer suggested and place a wall set in some distance from where the floor meets the ceiling. You wont waste this space if you put in lots of cupboards and drawers.
Some thing like this, although you needn't be this fancy.
I'd use 16mm, melamine coated, MR (moisture resistant) particle board or MDF for the major fit-out. It's what they use for kitchens, fairly tough, easy to clean and cheap. If you can create a detailled plan so you know the sizes that the boards needs to be cut to, most kitchen manufacturers will supply and cut board to size