Frank: what is your intent for this kart? Serious racing or just playing around? That may determine how much money and effort you want to put into the axle clutch, etc. What chassis are you building out? Where do you intend to race it?
May I ask why you want to use metric components? Unless you have these parts lying around then it would be less expensive and easier to use US parts like axle, bearings, etc.
Do you have your radiator/water cooling situation planned out?