By my way of thinking it seems best to buy three types. Small signal monolithics, medium power, and high power, with 25% going to P type and the rest N type. You'll want the small signal fets to be logic level, and maybe 25% of the power fets to be logic level. Not very common but depletion mode power fets have a place as well. Granted that's a lot of different types of fets, but you should cover your bases. Depends highly on what you think you'll be using the FET's for. I just covered what I think would be a general spread.