Plenty of table space, I use folding tables that I can pop up when needed. I use the same tables for market, I just make sure that the tables are good and clean for market.

Check with your regulations for a certified kitchen and make the walls and floors within their suggestions. You never know when rules will change. Build to specs or more, that way if things change you can do something else with it. Here that would mean, easy to clean walls and non-porous flooring. Plus a good sweep/mop cleans up easily.

Screens on windows and plenty of windows for the warmer days when it's rainy.

Randy, sounds like a great idea. I'd suggest making it bigger then you think. I ran out of room in mine pretty quick.
One think I especially like in our wash up room is a small RV type bath tub raised on a frame, it's about 2X4 and made of fiberglass. With a hose and sprayer, it's great to wash just about everything, with no bending over.

Other things I suggest: a concrete floor with drain, a rubber standing mat on the floor in front of the tub, lots of shelves and hooks for market baskets, permanent counter tops, bright lighting (inside and out), a 100 lb spring scale. And of course, a radio.

-Mark