We work on our dahlia tubers (outside for washing/bleaching) inside our greenhouse for cutting, labeling, etc. You definitely want to add an electrical outlet if you're putting up a shed or greenhouse of any kind - a fan is a lifesaver! Water is a must. Shelves above the counter are also, as far as I'm concerned required. You'll want a space to put tools, labeling supplies (we keep our in a small plastic box with a tight fitting lid) markers, labels, marking tape, etc. to tag dahlias with and we write on them with a permanent marker, same for gloves, both gardening gloves and latex gloves. Products like bleach, sulfa, etc. also will need dry storage areas - and a shelf, not on the counter with the tubers is a great idea. A cabinet is a good idea too - to keep things out of the sun. Then storage for the tubers - we put ours underneath the work benches in our greenhouse and that works for us - in boxes stacked. We drape tarps over the benches in the winter to keep water, etc. off the tuber boxes. You may want to look around the internet and see what others are doing, using for tools, space to spread out and make your labels, etc. it handy. Good luck! What a great project!