This shed will just have to be big enough - though my industrial machines are going to take up a big chunk of room. With the others, most will be on shelves with a few in cabinets. I can fit a lot of machines when I put them six high. I won't go higher than that with the heavy ones - don't want to hurt something (me or the machine) getting them down.
After the roof goes on, we'll wrap the outside with housewrap, put in the windows and side it. Then it will be sealed up, and we'll move to the inside. Then weather won't be a problem anymore. At some point I've got use the shed to move everything out of the summer kitchen next door and finish that up. That is the area that the cats will have access to (where their food and nice place to sleep is). They'll only get into the Machine Shed if I let them. Depends on if I need them for mice police. They have access to the other outbuildings and take their Patrol seriously.
I'm going to use radiant cove heating (gotta remember to order those heaters!) DH wanted baseboard as we have several of them on hand, but that would limit me for moving around cabinets. I plan on keeping the building above freezing in the winter, but probably not a lot more unless I'm working out there. Not sure about summer cooling, but our weather is normally on the dry side, and if I needed I can haul one of our portable AC units out there.
Next summer we'll fix up the exterior of the Summer kitchen, and then it will look nice. The plastic for the greenhouse roof was only 10 year material, and it's going to need to be replaced anyway.