Any engineers out there? Need a simple pallet-elevator

I feel the pulley system, your load distribution and your winch are the least of your concerns.

You need to figure out what the heaviest item you can safely store up there is. From that you can then go into attachment points for your pulley system. Your attachment points and methods also play a role in the weight you can lift and ultimately store above your space safely. Once those are figured out then you can get into the debate on winch, pulley system and your load distribution.