Mollie, sounds like a great project. My first bit of advice on building a shed or greenhouse is to make it half-again as large or twice as large if possible. At least built it so you can add on later.

I think cold water would be a great idea. PVC pipe is $1.50 per 10 feet section. At least buy fittings so you can attach the garden hose to the faucet from the outside. You can buy fiberglass laundry sinks for about $25. They have legs. You could set it up and just let it drain out on the ground. These would be deep enough for washing pots and general clean-up. Again, PVC pipe can be used as a drain. As long as you don't use too much soap or wash nasty stuff, you can let it run off into the yard. You can buy a spray on expanding foam insulation to spray between the studs, Just let it dry, shave it down to the same thickness as the studs and then cover the inside walls. What kind of heat will you use for your citrus? You will have to be very careful with any kind of heat and gasoline fumes from your mower, weedeater, chainsaw and many other gas powered tools. Good luck.

I'll second the advice about the size. Sheds tend to fill up quickly and before you know it you don't have enough room to turn around.

If you make it big, I would recommend dividing it into two parts with a dividing wall and two doors. That will let you put lawn stuff (mower, tiller, fertilizer, etc.) on one side and garden stuff on the other. You have more wall space for storage and don't have to climb over the lawnmower to get to your hoes. We have a 30+ year old 8 x 12 shed which the DH rehabbed with new sills and floor. It just has one door, but he put in a partial wall so I can have most of it for gardening but he can put his mower, etc. on the other side out of my way. He put up all sorts of hooks, shelves, and pegboard on each side, plus a magnetic strip for small tools and a work shelf on my side. Then he built a breezeway and another 8 x 12 shed (his playhouse ;-) ] for his saws, hammers, sawhorses, etc. and a work surface. He had to have a knee replaced before the finishing touches were done, but I'm loving my garden shed and he should be able to finish the rest soon.

Oh boy, a project after my own heart!
What kind of roof will it have? If it is a shed roof, make yourself a rain gutter ending in a rain barrel mounted up high; old timers down in Louisiana always had a big sistern out of cypress between the house and the kitchen garden. Plan to save as much rain as you can.

For the roof, if it is corrugated tin or something like that, you can get panels of the same corrugating pattern out of polycarbonate from Lowes/HD. They would allow light in too. It would only require a couple of panels. And if you do a cement floor, how about a slight slope toward the exit to help hose off the floor if you will store bags of mulch or fertilizer in there? And with pots of dirt around, it will have overflow from watering.

I'd put a potting bench on the outside wall where it would be convenient but not unsightly. A few brackets for hanging baskets and some heavy duty sconces for potted geraniums or something pretty lower down.

We have a 12x16 barn-shaped garden shed purchased from HD, and it looked huge when we got it. However, a riding lawntractor consumed most of the open space. You'd be surprised how FAST that stuff you mention storing in there takes up ALL the storage space. From the center ridge beam of our shed, we now have suspended my 14' canoe. My long handled shovels, hoes, rakes, etc, are in a long sort of broom closet attached to the privacy fence BEHIND the garden shed. Not even inside it. Of course, my dear hubby has cluttered up the shed so I have claimed the derelict cement block garage for my own. No roof on it currently, but when we put one on it, it will be the polycarbonate panels as much as possible. It already has a cement floor in very poor shape from tree roots pushing it up. However, the culprit tree was removed this month and that issue is under control.

What would be the sun orientation if you made a translucent leanto extending the shed at the rear? That would be a good way to winter your plants without rearranging your shed interior or reserving space for them all year. I have a book to recommend to you, which is new and great. As soon as I find it, I will post with the info.

Love your project. :)