How do you handle GUI buttons and image prefabs?

I'm working on an inventory system with 35 blocks. Five rows of 7. Each block will hold a single item image, or button image from inventory. Inventory is stored in List.

 List<Item>

I'm wondering about creating the buttons. Right now I have prefabs of sprites with a script attached with OnPointerClick event. This is different than my initial though of using GUI.[element] draws in OnGUI(). As you can see, I'm pretty confused. What is messing me up is, if I use sprite prefabs like I have set up now, they would have to be drawn in update, or OnGUI using Instantiate. I'm not sure of the best way to keep them from instantiating every Update call, so there aren't a million prefabs, yet the interface needs to be updated normally as items are picked up.

How would I do this, or how to you handle this situation?